Radiation detector with two-dimensional directionality

ABSTRACT

Disclosed is a directional gamma ray or neutron detector that locates a source both horizontally and vertically. In some embodiments, the detector comprises four “rod” scintillators around a shield, and an orthogonal “panel” scintillator mounted frontward of the rod scintillators. The azimuthal angle of the source may be calculated according to the detection rates of the rod scintillators, while the polar angle of the source may be calculated from the panel scintillator rate using a predetermined angular correlation function. Thus, the exact location of the source can be found from a single data set without iterative rotations. Embodiments of the detector enable rapid detection and precise localization of clandestine nuclear and radiological weapons in applications ranging from hand-held survey meters and walk-through portals, to vehicle cargo inspection stations and mobile area scanners. Such detectors are needed to detect clandestine nuclear weapons worldwide.

CROSS REFERENCES TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional PatentApplication No. 62/569,581 entitled “Gamma Ray Detector withTwo-Dimensional Directionality” and filed on Oct. 8, 2017, and U.S.Provisional Patent Application No. 62/580,960 entitled “Gamma RayDetector with Two-Dimensional Directionality” and filed on Nov. 2, 2017,and U.S. Provisional Patent Application No. 62/626,115 entitled“Directional Radiation Detector with Front Scintillator” and filed onFeb. 4, 2018, and U.S. Provisional Patent Application No. 62/661,072entitled “Radiation Detector with Two-Dimensional Directionality” andfiled on Apr. 22, 2018, the entire disclosures of which are incorporatedherein by reference.

FIELD OF THE INVENTION

The present invention relates generally to nuclear weapon detection.More particularly, the present invention is directed in one exemplaryaspect to a particle detector that determines a radiation sourcedirection in two dimensions.

BACKGROUND

Clandestine nuclear weapons are an immediate threat to every country andevery city in the world. A rogue nation with a nuclear weapon, or aterrorist group that acquires radiological material, could deliver it toa victim city via commercial shipping at low cost and low risk. Nuclearweapons are difficult to detect when shielded. Advanced radiationdetectors are necessary to reveal such weapons among backgrounds andbenign clutter. An urgent priority of the United States, and indeed ofall countries, is the development of radiation detectors that bothdetect and localize clandestine nuclear material.

A signature of all nuclear and radiological weapons is radiation,principally gamma rays (“gammas”) and neutrons. Gamma rays are detectedwhen they interact with matter via photoelectric absorption in which thegamma ray is absorbed and a photoelectron is emitted, Compton scatteringwhich generates a Compton electron and a scattered gamma ray, orelectron-positron pair production. In each case, the energetic electron(or positron, treated as an electron herein) can be detected in acharged-particle detector such as a scintillator. Neutrons are usuallyclassified according to energy as fast, intermediate, and slow. A fastor high-energy neutron, as used herein, has 100 keV to several MeV ofenergy. Fast neutrons can be detected by neutron-proton elasticscattering in which the recoil proton passes through a detector such asa scintillator. Slow or low-energy neutrons (1 eV or less, also calledthermal or epithermal) are detected by a capture reaction in aneutron-capture nuclide, usually ¹⁰B or ⁶Li, causing emission of promptions such as alpha and triton particles which can be detected in ascintillator or other ionization detector. Intermediate-energy neutronsmay be moderated or decelerated by multiple elastic scattering in ahydrogenous material such as HDPE (high-density polyethylene), and thendetected as slow neutrons.

Numerous directional radiation detectors have been proposed. Typically,they have one-dimensional directionality, meaning that they indicatewhether the source is to the left or right of the detector. Multipleiterative rotations are then needed to specify the source location inone dimension, such as the bearing of the source in a horizontal plane.This iterative rotation process is extremely time-consuming. Inaddition, a one-dimensional scan is not sufficient to localize a threatin large inspection items such as trucks and railcars and shippingcontainers; a two-dimensional determination is needed. Although a pairof one-dimensional directional detectors might conceivably be used toseparately scan horizontally and vertically, this would require twoseparate systems and would entail some kind of coordination betweenthem. Also, the two systems would each have their own background rate,further diluting the threat signature and requiring longer scan times.Alternatively, a single prior-art directional detector might conceivablybe able to scan horizontally first, then roll by 90 degrees, and thenscan vertically, but this would take twice as long and would require acomplicated mechanical joint.

Prior art further includes ostensibly directional gamma ray detectors(U.S. Pat. No. 8,319,188 to Ramsden, U.S. Pat. No. 7,944,482 to Frank,for example) comprising four scintillators packed around a detector axisand analyzed to determine an azimuthal angle and, in the case of theRamsden device, a low-resolution indication of the polar angle as well.These configurations lack a central shield and thus must depend on thevarious scintillators to shield each other, which greatly limits theangular contrast achievable.

What is needed, then, is a gamma ray or neutron detector system withtwo-dimensional directionality, that preferably provides superiorangular resolution extending all the way around the detector includingpolar angles near the midplane of the detector, and with enoughsensitivity to detect a shielded source. Preferably the detector wouldindicate the direction toward the source, in two dimensions, using asingle data set acquired at a single orientation of the detector,thereby avoiding the need for iterative rotations. Preferably such adetector would be compact, fast, efficient, easy to build, easy to use,and low in cost.

SUMMARY

Disclosed herein are systems for nuclear weapon detection. In oneembodiment, a directional detector device (the “detector”) can beconfigured to detect particles from a radioactive source, and todetermine the direction of the source in two dimensions, such as theazimuthal and polar angles of a spherical coordinate system. FIG. 1shows how the polar and azimuthal angles are related to a directionalmeasurement. In one embodiment, the directional detector determines thesource location from scintillator detection data acquired at a singleposition and a single orientation of the detector. The examples aredirected to particles comprising gamma rays, fast neutrons, and slowneutrons, but the principles disclosed herein are readily applicable toany particle type.

Some embodiments of the detector comprise a shield, a “panel”scintillator, and N “rod” scintillators, where N is equal to three ormore (in preferred embodiments, N=4). The panel and rod scintillatorsmay be configured to detect the source particles according tointeractions by the source particles in the various scintillators. Theinteractions can generate “secondaries” comprising energetic chargedparticles such as gamma-generated electrons, recoil protons, or ionsemitted following neutron capture, and those charged particles producescintillation light, which may then be detected by a light sensor. Thelight sensor may responsively produce an electronic signal related tothe scintillation light, and a processor may then analyze those signalsand calculate the azimuthal and polar angles of the source. The detectorhas a “detector axis” which is an axis that extends from the back to thefront of the detector and passes through the center of the detector. Insome embodiments, the rod scintillators comprise elongated prism-shapedbodies, positioned symmetrically around the detector axis, and orientedwith their longest dimension parallel to the detector axis. In someembodiments, the panel scintillator may be a slab-shaped scintillatororiented perpendicular to the detector axis, and may be positionedfrontward of the rod scintillators. In some embodiments, the shieldcomprises N plates of shielding material thick enough to block orattenuate most (i.e., over 50%) of the source particles incidentorthogonally on the shield plate. In some embodiments, the detector maybe configured to calculate the azimuthal angle of the source byanalyzing particle detection data, such as counting rates, in thevarious rod scintillators. In some embodiments, the detector may also beconfigured to calculate the polar angle of the source by analyzingparticle detection data in the panel scintillator, using, for example, apredetermined angular correlation function that relates the polar angleto the various scintillator counting rates.

In some embodiments, the various shield plates are arranged in aradially-oriented array wherein each shield plate abuts or joins to allthe other shield plates at or near the detector axis. Alternatively,when N is an even number, the shield may comprise N/2 plates thatmutually intersect at the midline of each plate. In some embodiments,the shield extends frontward to the panel scintillator and maysubstantially abut the back surface of the panel scintillator. In otherembodiments, a space may be provided between the panel scintillator andthe shield. In some embodiments, the shield is configured tosubstantially prevent the source particles and their secondaries frompassing between the rod scintillators. More specifically, the shieldplates prevent particles from passing out of any one of the rodscintillators and into any other rod scintillator, thereby isolating thevarious rod scintillators from each other. In some embodiments, the rodscintillators and the panel scintillator are configured to surround oralmost surround the shield, so that only the edges of the shield platesare exposed to the outside. In such a configuration, the sourceparticles cannot reach the shield (other than the edges) without firstpassing through at least one scintillator. As a result, substantiallyall of the particles impact scintillator material before reaching anyshield material, resulting in few particles lost in shielding materialand thus a high overall detection efficiency.

In some embodiments, the shield may protrude, or extend beyond, the rodscintillators in the frontward direction. Such a protrusion may sharpenthe angular resolution by preventing particles that pass in front of theshield from reaching the downstream rod scintillator. Particles arrivingat an oblique angle and striking the downstream rod scintillator canpartially dilute the angular data, reducing the angular contrastachievable. The protrusion distance is preferably sufficient to blockparticles arriving at large polar angles, such as polar angles greaterthan 30 degrees, 45 degrees, or 60 degrees. Therefore, in someembodiments, the protrusion distance is related to the lateraldimensions of the rod scintillators, where a “lateral dimension” is asize measured perpendicular to the detector axis. For example, in someembodiments, the protrusion distance may be equal to 0.5 times one ofthe lateral dimensions of the rod scintillators, or 1.0 times theaverage of the two lateral dimensions, or other function of the lateraldimensions of the rod scintillators.

In some embodiments, the shield may be truncated, or cut short, at theback end, so as to reduce the weight of the detector for example. Stateddifferently, the rod scintillators may extend in the rearward directionsubstantially beyond the shield. In many applications, the source isexpected to be in the front half-space of the detector, in which casethe rearmost portion of the shield has practically no effect on theparticle trajectories. Therefore, in some embodiments, the rearmostportion of the shield may be truncated without significantly impactingthe performance of the detector.

In some embodiments, the shield may be tapered to reduce the weightwhile still retaining some shielding effect from front to back. Forexample, each shield plate may be tapered in thickness, being thicker atthe front and thinner at the back of the detector. Tapering in this waycould reduce the shield weight by 50% while having little effect on thesource location determination.

In some embodiments, each rod scintillator may be a solid body shaped asa right prism, which may be beveled or tapered or shaped in variousways. The rod scintillators may have a cross-sectional shape (oftencalled the “base” of the prism) which is typically a square or arectangle or a triangle or a pie-sector or an arcuate shape. The baseshape may then be extruded along an extrusion axis. Each rodscintillator may be positioned in the angular openings between theplates of the shield, adjacent to two of the shield plates respectively.Likewise, each shield plate may be positioned between and adjacent totwo of the rod scintillators, respectively. In some embodiments, eachrod scintillator may be oriented with its longest dimensionperpendicular to the panel scintillator. Usually the longest dimensionis the extrusion axis of the prism shape. In such embodiments, the rodscintillators may be parallel to the detector axis while the panelscintillator is perpendicular to the detector axis. In otherembodiments, the extrusion dimension may be less than the lateraldimensions, or the extrusion dimension may be substantially equal to thelateral dimensions (for example, a cube-shaped rod scintillator).According to some embodiments, the angular sensitivity of the rodscintillators is opposite to the angular sensitivity of the panelscintillator. Specifically, in some embodiments, each rod scintillatoris mainly sensitive to particles arriving from one side, due to thepresence of the shield, whereas the panel scintillator is mainlysensitive to particles arriving from the front. The polar angle may thenbe calculated by exploiting those contrasting angular sensitivitydistributions, as discussed in detail below. Each rod scintillator maybe positioned in the angular openings between the plates of the shield,adjacent to two of the shield plates respectively. Likewise, each shieldplate may be positioned between and adjacent to two of the rodscintillators, respectively.

In some embodiments, the panel scintillator may be a slab-shaped bodyoriented perpendicular to the detector axis, positioned frontward of therod scintillators, and centered on the detector axis. The panelscintillator may be thick enough to detect all, or substantially all, ofthe particles orthogonally incident on the panel scintillator, while therod scintillators may detect particles that pass beside the panelscintillator as well as particles that scatter in the panel scintillatorand then pass through to the rod scintillators. Alternatively, the panelscintillator thickness may be thin enough that most of the orthogonallyincident particles pass through the panel scintillator and can then bedetected in the rod scintillators, yet thick enough to provide asufficient detection efficiency for determining the polar angle of thesource. A particle “passes through” the panel scintillator when theparticle exits the back surface of the panel scintillator with enoughenergy to be detected in a rod scintillator. For example, a particle maypass through the panel scintillator without interacting, or the particlemay scatter and be detected in the panel scintillator and then go on tobe detected again in the rod scintillator. In some embodiments, thepanel scintillator thickness is such that at least 10% of theorthogonally incident source particles are detected in the panelscintillator, while over 50% of the orthogonally incident particles passthrough the panel scintillator (possibly with some scattering) and arethen detectable in the rod scintillators. These values ensure that thepanel scintillator detects enough particles to provide a measure of thepolar angle, while not blocking the rod scintillators from receivingenough particles to provide a measure of the azimuthal angle.

In some embodiments, the panel scintillator may enable the detector tocalculate the polar angle precisely. The panel scintillator may have anangular sensitivity distribution that is substantially opposite to theangular sensitivity distribution of the rod scintillators, so thatparticles from a discrete source produce different detection rates inthe panel and rod scintillators depending on the polar angle of thesource. According to some embodiments of the panel scintillator, thelateral dimensions of the panel scintillator may each be at least twotimes, and more preferably three times or four times, the thickness ofthe panel scintillator. In some embodiments, the lateral dimensions maybe substantially larger, such as 10 or 20 times the thickness of thepanel scintillator. Due to its thinness, in some embodiments, the panelscintillator may be mainly sensitive to particles arriving from thefront, while being substantially less sensitive to particles arrivingfrom the midplane, since the panel scintillator is “edge-on” to theparticles arriving from the side. In some embodiments, this angularsensitivity distribution is opposite to the angular sensitivities of therod scintillators, which are mainly sensitive to particles arriving fromone side. The processor may employ that sensitivity contrast tocalculate the polar angle from the various scintillator particledetection rates.

In some embodiments, the thickness of the panel scintillator may besubstantially less than an average interaction distance of the particlein the panel scintillator material, while the panel scintillator lateraldimensions may both be substantially greater than the particle averageinteraction distance. The “average interaction distance” of a particleis the distance that the particle would travel in a particular material,on average, before interacting in a way that would cause the particle tobe detected. For gamma rays, the average interaction distance is themean free path for Compton scattering or photoelectric absorption orpair-production, or alternatively is the inverse of the mass attenuationfactor which includes all those interaction types. For fast neutrons,the average interaction distance is the mean free path for n-pscattering. For slow neutrons, the average interaction distance is themean free path for neutron capture. In some embodiments, the panelscintillator may be configured so that each lateral dimension of thepanel scintillator is at least two times the average interactiondistance, while the thickness is at most 0.5 times the averageinteraction distance (hence the average interaction distance is at leasttwo times the thickness of the panel scintillator).

In some embodiments, the panel scintillator may be shaped like theshield in transverse cross-section, that is, with N arms extending fromthe center. The panel scintillator so shaped may substantially match theN plates of the shield, and thus can reside directly over the shieldplates according to some embodiments. One advantage of shaping the panelscintillator in this way is that when the detector axis is aimeddirectly at the source, the rod scintillators can be completelyunobscured by the shaped panel scintillator.

In some embodiments, the detector may further include a second panelscintillator, which may be identical to the first-mentioned panelscintillator but positioned behind, or rearward of, the rodscintillators. In such a double-ended configuration, the first andsecond panel scintillators may both be perpendicular to the detectoraxis. In some embodiments, the double-ended detector may be configuredto compare the detection rates in the first and second panelscintillators to determine whether the source is in the front or backhalf-space. Then whichever one has the higher counting rate can be usedin the polar angle calculation. Alternatively, a source-location fittingprogram may use data from all the scintillators to determine the polarand azimuthal angles. In this way, the double-ended configuration canview a full 4π of solid angle everywhere around the detector, and thusdetermine the source location everywhere around the detector includingthe midplane, directly behind and directly in front of the detector, andeverywhere else.

The rod scintillators may be shaped or beveled to reduce theirsusceptibility to particles that arrive from an oblique angle such as30, 45, or 60 degrees relative to the detector axis. Particles that passover the shield and then strike the downstream rod scintillatorrepresent an erroneous event that dilutes the angular sensitivity, asmentioned. To avoid this, the rod scintillators may be cut back at anangle, or beveled, to remove scintillator material farthest from theshield axis.

In particular embodiments, (a) the lateral dimensions of the panelscintillator may substantially match the lateral extent of the rodscintillator array, or (b) the panel scintillator may extend laterallybeyond the rod scintillators, or (c) the panel scintillator may besmaller than the rod scintillator array. Each such configuration hasadvantages. An extended panel scintillator that extends laterally beyondthe rod scintillators may have extra detection efficiency due to itslarger size. A panel scintillator that matches the rod scintillatorarray size can provide a tidy and compact structure which is easy tofabricate and easy to mount in a holder or case. A smaller panelscintillator may leave part of the rod scintillators unobstructed toparticles arriving from the front, thereby enhancing the rodscintillator detection efficiency.

The panel scintillator may comprise a single monolithic body orientedperpendicular to the detector axis and positioned frontward of the rodscintillators. Alternatively, the panel scintillator may be divided intoN portions which are each slab-shaped and oriented perpendicular to thedetector axis and positioned frontward of the rod scintillators. In someembodiments, the panel scintillator portions may be shaped as smallslabs. Each of the panel scintillator portions may be positionedadjacent to, and frontward of, one of the rod scintillatorsrespectively, and each rod scintillator may be adjacent to, and rearwardof, one of the panel scintillator portions respectively. In addition,the shield may protrude frontward of the panel scintillator portions, toimprove the angular resolution for example.

When the panel scintillator is divided into portions, each such panelscintillator portion may be optically isolated from the adjacent rodscintillators, in which case each panel scintillator portion may beviewed by a separate light sensor respectively, and each rodscintillator may be viewed by a separate light sensor respectively. Thishas the advantage of simplicity since each scintillator provides aseparate signal on a separate conductor. In another embodiment, eachpanel scintillator portion may be optically coupled to the adjacent rodscintillator, and each rod scintillator may be optically coupled to oneof the panel scintillator portions respectively. In that case, thecoupled scintillators may be viewed simultaneously by a shared lightsensor. Such optically coupled panel and rod scintillators preferablycomprise different scintillator materials that produce detectablydifferent pulses, such as differently shaped light pulses or differentwavelength pulses.

As a further option, the shield or a portion of the shield may be madefrom a material that is transparent to the light emitted by the panelscintillator, and can thereby serve as a light guide for the panelscintillator. Such a transparent shield (or shield portion) may beoptically coupled to the panel scintillator and to a light sensoraccording to some embodiments. Preferably, the transparent shieldportion provides sufficient shielding property, such as blocking orattenuating over 50% of the particles orthogonally incident thereon. Oneadvantage of such a light-guide-shield is that it can allow all of thelight sensors to be mounted on the back surface of the detector, therebykeeping the light sensors out of the way of incoming particles from thefront.

As a further option, the shield (or a portion thereof) may comprise ascintillator as well as a shield. A scintillating shield for gamma raysmay comprise a high-Z, high-density scintillating material such as BGO,LYSO, LuAP, or CdWO₄ for example. In addition, the scintillating shieldmay also be a spectroscopic-type detector such as NaI or LaBr₃ or otherspectroscopic scintillator. A “spectroscopic” type scintillator measuresthe total energy of the particle, preferably with an energy resolutionof 10% or better, and thus helps to identify the isotopic content of thesource. One or more light sensors may then be coupled to thescintillating shield. One advantage of a scintillating shield is that itcan provide an additional, high-sensitivity measure of the radiationbackground. In addition, if the scintillating shield is spectroscopic,signals from the scintillating shield may reveal the isotopic content ofthe source according to the energies of the particles (usually gammas)detected by the spectroscopic shield.

In some embodiments, a shaped “shield slug” may be mounted frontward ofthe panel scintillator. The shield slug may be shaped similarly to theshield itself in cross-section, and configured to block or attenuatemost of the particles orthogonally incident thereon. The shield slug maythus serve a similar function as a shield protrusion, blocking particlesthat arrive at oblique angles to the detector axis and preventing thoseparticles from striking the downstream rod scintillator.

Typically, each of the panel and rod scintillators, and a second panelscintillator if present, (collectively, “the scintillators”) may beconfigured to emit a light pulse when traversed by a charged particlesuch as a gamma-generated electron or a neutron-generated ion. Eachscintillator may be connected directly, or through a light guide, to alight sensor, which is a transducer such as a photomultiplier tube or aphotodiode, configured to produce an electronic signal responsive toeach scintillator light pulse. In some embodiments, the electronicsignals from particle interactions in each scintillator are distinct, sothat the particular scintillator associated with each signal can bedetermined. For example, the signals may appear on separate conductorsor may have different pulse properties, so that the processor candetermine which scintillator was involved. Optionally, the panelscintillator and/or rod scintillators may comprise a spectroscopic typescintillator that measures the energy of the particles and thus helpsidentify the source composition.

In some embodiments, the detector may include a processor comprisingdigital, and optionally analog, electronics configured to readinstructions from a non-transient computer-readable medium. Theinstructions, when executed by the processor, cause the processor toperform a method that includes analyzing the electronic signals from thelight sensors, determining which scintillator produced each signal,accumulating detection data for each scintillator during a timeinterval, and thereby determining particle detection rates orinteraction tallies for each of the scintillators. The method may theninclude subtracting each rod scintillator rate from that of thediametrically opposite rod scintillator, thereby determining adifferential for each rod scintillator, and then calculating the sourceazimuthal angle by analyzing the rod scintillator data or thedifferentials. The processor may use weighted averaging or interpolationor a fitting function or a variable source model or other analysis stepsto determine the azimuthal angle of the source from the detection dataof the rod scintillators. The method may further include determining thepolar angle of the source, for example by calculating a function of thedifferentials divided by the panel scintillator detection rate, therebyobtaining a ratio, and comparing the ratio (or its inverse) to apredetermined angular correlation function which yields the polar angleof the source.

In some embodiments, the panel scintillator is centrally mounted and isunshielded, and therefore has a symmetrical angular sensitivity. The rodscintillators, on the other hand, can be mainly sensitive to particlesthat arrive from one side, since the shield blocks particles arrivingfrom the other side. Therefore, according to some embodiments, each rodscintillator may have a strongly antisymmetric angular sensitivitydistribution. The detector may exploit these contrasting angularsensitivities to derive a unique formula that yields the polar angle. Insome embodiments, the azimuthal and polar angles may be calculated fromdetection data acquired at a single orientation of the detector. Thecorrelation may provide precise values for the azimuthal angle from 0 to360 degrees, and for the polar angle from zero to 90 degrees (that is,from straight-ahead to the midplane of the detector). For thedouble-ended version of the detector, polar angles may be determinedfrom zero to 180 degrees, thereby encompassing the entire sphere,without rotating or moving the detector. This truly omnidirectionalcapability is in contrast to prior-art detectors that provide polarangle determination only for small polar angles in the fronthalf-sphere.

The processor may be further configured to compare the particledetection data in the various rod scintillators and to determine, whenthey are all substantially the same, that the detector axis issubstantially aligned with the source. The processor may be furtherconfigured to refine the polar angle determination based on theremaining differences between the various rod scintillator countingrates.

Optionally, the detector may include a light beam emitter, such as alow-power laser pointer or flashlight. In a first embodiment, the lightbeam may be fixedly aligned with the detector axis, thereby illuminatingthe aim point of the detector. In preferred embodiments, the shape orother property of the light beam may be varied according to thecalculated azimuthal and polar angles of the source, thereby causing thebeam spot shape to indicate or point toward the source location inreal-time. For example, in some embodiments, the light beam may beshaped as a wedge or arrow, pointing toward the source azimuthal angle,and with a length or shape indicative of the polar angle. In a secondembodiment, the light beam is not aimed along the detector axis, butrather, is redirected so as to point directly at the calculated sourcelocation using, for example, a rotatable mirror that is driven accordingto the calculated azimuthal and polar angles, thereby bathing thesuspected source location with the light beam. The redirected beam mayalso be modulated to indicate the radiation level or the type ofparticle detected or other information. Alternatively, the beam may bemade wider or narrower according to the uncertainty in the sourcelocation. The operator of the detector can then learn a great deal ofinformation about any source detected simply from the light beam inreal-time, without having to look away from the scene.

Optionally, the detector may include imaging means, such as a still orvideo camera. In a first camera embodiment, the camera is aligned withthe detector axis, and is configured to record the scene centered on thedetector axis. In a second camera embodiment, the camera is configuredto center the image on the calculated source location, thereby recordingthe detected source along with surrounding items. Such a source-centeredimage has an advantage that the image fully views the source location,rather than the aim point of the detector which may or may not berelevant. Also, the image which is centered on the source location maybe easily magnified using a zoom lens, for example. By varying the zoomlens, successive images can be acquired, from a wide-angle view all theway to a telescopic close-up view of the source location, without havingto readjust the aim point. The camera may be activated to acquire theimage as soon as the source location is determined, when the radiationlevel exceeds background levels, when the detector is substantiallyaimed at the source according to the rod scintillator rates being equal,or manually by the operator, or whenever the detector is rotated. Insome embodiments, the source location and other information may besuperposed upon the camera image. If the detector is subsequentlyrotated, the rotation angle may be determined by image analysis ofimages taken before and after the rotation.

The detector may include non-visual indicators such as sonic or hapticindicators. The non-visual indicators may be activated according to thecalculated azimuthal or polar angles, thereby assisting the operator inlocalizing the source.

Background events such as cosmic rays, and complex events such asmultiscattering events, may be excluded by signal processing andelectronic logic according to some embodiments. For example, any eventin which more than one rod scintillator is triggered at the same timecan be rejected. Pulses that are too large to be produced by theparticles of interest can also be rejected using a second discriminationthreshold set just above the gamma ray or neutron energy, therebyeliminating most cosmic rays.

Various embodiments provide many advantages over prior-art directionaldetectors. (a) The detector can determine both the azimuthal angle andpolar angle of the source, thereby locating the source in twodimensions. (b) The detector can determine the source location usingonly a single acquisition of scintillator data at a single orientationof the detector, thereby avoiding extensive iteration and rotations. (c)By use of the panel scintillator detection rates, the detector canprovide high-resolution polar angle determination, for source anglesthroughout the front half-sphere from the detector axis to the midplaneand, with the addition of a second panel scintillator, can cover theentire 4π sphere. (d) The detector can specifically detect when thedetector axis is aligned with the source, by comparing the rodscintillator signals. (e) The detector can provide high detectionefficiency since, in some embodiments, the scintillators nearly surroundthe exterior surface of the shield, thereby presenting maximum detectionarea with very few source particles lost to the shield material. Thisalso can ensure particle detection from all directions at all times. (f)Various embodiments can detect all three major particle types forsecurity inspections—gamma rays, fast neutrons, and slow neutrons—bysubstituting appropriate scintillator and shield materials. (g) Thedetector can be low in weight due to the shield being configured not forcollimation, but only for isolating the rod scintillators from eachother. (h) The detector can be compact, due primarily to the placementof the scintillators in close proximity to the shield, therebyminimizing the overall envelope of the system. This may also greatlyenhance the angular performance. (i) Various embodiments of the detectorare economical, easy to build, easy to use, and require noexotic/rare/expensive materials. (j) The detector may be suitable forcritically important security applications including as a portablesurvey instrument, a walk-through portal, a fixed-site cargo and vehiclescanner, and a mobile area scanner for concealed weapons. (k) By raisingan alarm when several particles are detected coming from the samelocation, various embodiments can provide greatly improved speed andsensitivity compared with conventional non-directional detectors. Thedetector can thereby defeat any attempt to obscure a concealed weaponwith shielding and obfuscation.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a perspective sketch showing how the azimuthal and polarangles are related to the detector axis and the source location.

FIG. 2 is a perspective sketch of an exemplary directional detectoraccording to the present disclosure with four rod scintillators.

FIG. 3 is a rear-view perspective sketch of the exemplary directionaldetector of FIG. 2 but with light sensors and a larger panelscintillator.

FIG. 4 is an exploded perspective sketch of the exemplary directionaldetector of FIG. 2 with shield plates, rod scintillators, and panelscintillator all separated.

FIG. 5 is a perspective sketch, partially exploded, of the exemplarydirectional detector with tapered shield plates.

FIG. 6 is a perspective sketch of the exemplary directional detectorincluding shield truncation, shield protrusion, and a panel scintillatorseparated into portions.

FIG. 7 is a perspective sketch, partially exploded, of the exemplarydirectional detector with beveled rod scintillators, a shieldprotrusion, a monolithic panel scintillator, and a shield slug.

FIG. 8 is a perspective sketch, partially exploded, of an exemplarydirectional detector in which the panel scintillator is shaped like theshield so as to leave the rod scintillators unobstructed from the front.

FIG. 9 is a perspective sketch, partially exploded, of an exemplarydirectional detector with triangular shaped rod scintillators, and twooptions for mounting the panel scintillator.

FIG. 10 is a perspective sketch, partially exploded, of an exemplarycylindrical version of a directional detector.

FIG. 11A is a perspective sketch of an exemplary double-ended version ofa directional detector

FIG. 11B is a perspective sketch of the exemplary version of FIG. 11Awith different light sensors.

FIG. 12A is a longitudinal cross-section sketch of an exemplarydirectional detector with a transparent shield optically coupled to thepanel scintillator, and beveled rod scintillators.

FIG. 12B is a transverse cross-section sketch of an exemplarydirectional detector showing the transparent shield portion in relationto the shield plates.

FIG. 13 is a perspective sketch, partially exploded, of an exemplarydirectional detector with three rod scintillators and two panelscintillators front and back.

FIG. 14 is a cross-section sketch, partially exploded, of an exemplarydirectional detector with hexagonal symmetry and six rod scintillators.

FIG. 15 is a cross-section sketch of an exemplary directional detectorconfigured with rectangular rod scintillators to provide high angularresolution in one direction, and high detection efficiency in anorthogonal direction.

FIG. 16 is a flowchart showing steps of an exemplary method forcalculating the polar and azimuthal angles from the scintillator datawithout rotations.

FIG. 17 is a flowchart showing steps of an exemplary method for rotatingthe detector into alignment with the source.

FIG. 18 is a graph showing the scintillator counting rates for an MCNP6simulation of a gamma ray detector.

FIG. 19 is a graph showing the angular correlation function that relatesthe polar angle to the scintillator counting rates for the simulation ofFIG. 18.

FIG. 20 is a perspective sketch of an exemplary hand-held survey meterincorporating a directional detector.

FIG. 21 shows an exemplary display using LEDs to indicate the azimuthaland polar angles to the source.

FIG. 22 depicts an exemplary flat-screen display showing the directionof the source and the magnitude of the polar angle.

FIG. 23 shows multiple renditions of a light beam spot configured topoint toward the source and also to indicate the polar angle of thesource.

FIG. 24 is a notional perspective sketch of an exemplary array ofdirectional detectors arranged to scan passing pedestrians forradioactive material.

FIG. 25 shows in perspective an exemplary mobile area scannerincorporating double-ended directional detectors.

FIG. 26 shows an exemplary vehicle scanner system incorporating arraysof directional detectors.

FIG. 27 shows an alternative vehicle scanner in which the directionaldetectors are added to a prior-art cosmic ray inspection system.

DETAILED DESCRIPTION OF INVENTION

In the following description, reference is made to the accompanyingdrawings in which it is shown by way of illustration specificembodiments in which the invention can be practiced. Not all of thedescribed components are necessarily drawn to scale in order toemphasize certain features and to better facilitate the reader'sconception of the disclosed embodiments. It is to be understood thatother embodiments can be used and structural changes can be made withoutdeparting from the scope of the embodiments of disclosed herein.

A directional particle detector, configured to determine the azimuthaland polar angles of a radioactive source location relative to thedetector axis, may detect any type of source and any type of particle,but the examples and applications disclosed herein are primarily appliedto gamma ray and/or neutron detection. In some embodiments, the detectorcan determine the azimuthal angle of the source according to theparticle detection rates of the rod scintillators, and can determine thepolar angle of the source by comparing a ratio of the panel scintillatorrates to a predetermined angular correlation function. In someembodiments, the detector can determine the source location in twodimensions, from a single set of scintillator data, which can beacquired at a single position and a single orientation of the detector.Specifically, the detector can localize the source anywhere throughoutthe front half-space of polar angles of 0 to 90 degrees, or with frontand back panel scintillators can cover the entire 4π space includingazimuthal angles of 0-360 degrees and polar angles of 0-180 degrees withhigh precision throughout.

In some embodiments, the detector can be adapted to detect gamma rays,fast neutrons, or slow neutrons by adjusting the compositions of theshield and scintillators. The properties of the light sensors and theprocessor, and particularly the analog electronics, can also be adjustedaccording to the scintillator choices. For detecting gamma rays, thescintillators may be any of the economical plastic scintillator typessuch as common PVT-based plastic, or stilbene, or other polymer. Morepreferably, the panel scintillator is a higher-density material such asLYSO, BGO, CdWO₄, LuAP, CsI, or NaI, among many other possibilities. Thepanel and rod scintillators may all comprise the same material, or thepanel scintillator may be different from the others according to someembodiments. Preferably the rod scintillators are all identical incomposition and shape, for ease of calculating the azimuthal angle. Theshield, adapted to gamma rays, may be any high-Z, high-density material(Z being the atomic number), such as lead, tungsten, bismuth, or thelike, and sufficiently thick to substantially prevent gamma rays frompassing from any rod scintillator to any other rod scintillator. Thegamma ray shield may alternatively comprise lower-density materials suchas steel, or a mixture of materials such as leaded glass, or layers suchas polyethylene coated with lead and LiF, provided that the shieldplates are thick enough to block or attenuate over 50% of the gamma raysincident orthogonally on the shield plate.

For detecting fast neutrons, according to some embodiments, thescintillators may be a hydrogenous material that provides abundant n-pscattering targets. Common plastic scintillators detect fast neutrons inthis way, but they are also sensitive to gammas, which may be a problemin applications where background gamma rays greatly outnumber the threatneutrons. Therefore, according to some embodiments, the scintillatorsmay comprise stilbene, or may include a special fluor that emitsdifferent shaped light pulses depending on the ionization density of theparticle (also called “PSD scintillators”, standing for pulse-shapediscrimination). Certain inorganic scintillators also have thisionization-dependent pulse shape property and, if blended or layeredwith a hydrogenous matrix, can detect n-p scattering selectively.Currently the obtainable separation in pulse shape is not large, butimproved PSD scintillator materials are being developed. Alternatively,the scintillators may comprise a material such as ZnS, which produceslittle or no light for gamma-generated electrons which have a lowionization density, but a very large light pulse for recoil protons orneutron-capture ions which have a very high ionization density for ashort track. Such intrinsically gamma-blind scintillators may beembedded in or adjacent to a hydrogenous material such as an acrylic orother transparent polymer matrix that provides n-p scattering targets.The transparent matrix may include a wavelength shifter for improvedlight propagation according to some embodiments. The shield, adapted tofast neutrons, is preferably HDPE or paraffin, or other material with ahigh density of hydrogen, and is sufficiently thick to degrade theenergy of the fast neutrons sufficiently that they are no longerdetectable in scintillators by n-p scattering.

For detecting slow neutrons, the scintillators may comprise any of theabove listed scintillators, combined with a neutron-capture nuclide suchas ¹⁰B or ⁶Li. Ions emitted from the neutron capture can then excite theadjacent scintillator. The capture nuclide may be in a particulate formdistributed in a scintillating matrix, or it may be a thin film coatedonto a scintillator, or other forms that enable the reaction ions topass into the scintillator. Alternatively, the scintillator couldcomprise the neutron-capture nuclide in its composition such as LiI(Eu).However, LiI is sensitive to gammas, as are most of the scintillatortypes currently available. The shield for slow neutrons is preferablyHDPE or paraffin, loaded or coated with a neutron-capture material. Theneutron-capture nuclide is preferably lithium if the scintillators aresensitive to gamma rays, since lithium produces negligible gammabackground upon neutron capture. But if the scintillators aregamma-blind, then any neutron-capture material such as boron orgadolinium may be suitable. According to some embodiments, the amount ofsuch neutron-capture material is preferably sufficient to capture anyslow neutrons that pass through one of the rod scintillators, andthereby prevent that neutron from going into another rod scintillator.

In an embodiment, the processor can accumulate counts from the variousscintillators for a specific time interval termed the integration time,and then calculate the polar and azimuthal angles from the accumulatedparticle detection data. Optionally, the integration time may bemanually variable, so that the operator can select a short integrationtime for a quick initial indication of the source direction, and then alonger integration time to obtain a more precise result. Or, theintegration time can be adjusted automatically, based on scintillatorcounting rates for example. In a high-radiation environment with highcounting rates, the processor can select a short integration time,thereby obtaining precise azimuthal and polar angles quickly, andthereby reduce operator exposure in the hazardous environment. If thesource is faint or well-shielded, the low counting rates may require alonger integration time to provide a reliable detection.

The detector may include an indicator or display for visually reportingthe calculated source location in two dimensions. In some embodiments,the indicator or display shows both the azimuthal angle and the polarangle of the source relative to the current detector axis. The indicatormay comprise a circle (or a plurality of concentric circles) of LEDs(light emitting diodes) or other luminous components, and a particularLED may be illuminated so as to indicate the calculated azimuthal andpolar angles of the source. For example, the display may comprise asingle circular array of LEDs of which one particular LED isilluminated, thereby indicating the azimuthal angle, and that LED may beflashed or otherwise modulated to indicate the size of the polar angle.In some embodiments, the illuminated LED may be steady when the polarangle is large, and may be flashed or modulated increasingly rapidly asthe detector axis approaches the source location. In another embodiment,multiple concentric circles of LEDs may be provided, each circlecorresponding to small, medium, and large polar angles for example. Insome embodiments, a specially-colored central LED may be provided andilluminated only when the detector axis is aligned with the source. TheLED display is easy to learn and use, enabling inspectors to rapidlylocate radioactive sources using, for example, a hand-held detector.

In another embodiment, the indicator may comprise a flat-screen displaythat shows an arrow or other rotatable icon, arranged to point at thecalculated azimuthal angle, thereby pointing toward the source. The iconmay be varied to show the magnitude of the polar angle, for example byshowing a longer or brighter or distinctively colored arrow depending onwhether the polar angle is large or small. In some embodiments, the iconmay be flashed or otherwise modulated according to the size of the polarangle. In some embodiments, the polar angle may be indicated separatelyfrom the rotatable icon, using for example a bar widget on the screen.In addition, or alternatively, the screen may show numerical values (indegrees, for example) corresponding to the calculated azimuthal and/orpolar angles. The rotatable icon may be changed to a special alignmentindicator when the detector axis is aimed at the source, such as aprominently flashing non-directional icon, or a flashing light or atone.

To calculate the azimuthal angle of the source, the detector may beconfigured to analyze the rod scintillator detection rates or otherequivalent data related to particle interactions in the rodscintillators. The detector may then calculate the azimuthal angle ofthe source relative to the position angle of each rod scintillator. The“position angle” of each rod scintillator is the azimuthal angle of thecentroid of that rod scintillator, measured clockwise around thedetector axis as viewed from the rear, and starting at an arbitrary zeroposition such as the horizontal right-hand side as viewed from the rear.In some embodiments, the detector may calculate the azimuthal angle byinterpolating between the position angles of the particular two rodscintillators that have the highest counting rates. Alternatively, thedetector may use weighted averaging, numerical fitting of the rodscintillator detection rates using a source model in which the azimuthalangle is variable, or another suitable method for calculating the sourceazimuthal angle from the rod scintillator data.

Alternatively, the detector may be configured to calculate adifferential for each rod scintillator, each differential being equal tothe difference between the counting rate of each rod scintillator minusthe counting rate of the diametrically opposite rod scintillator. If thedetector has an odd number of rod scintillators, then the differentialequals the rod scintillator detection rate minus the average of thedetection rates of the two opposing rod scintillators. In someembodiments, the detector then calculates the azimuthal angle bylinearly interpolating between the two highest differentials.Alternatively, the detector may be configured to calculate the azimuthalangle by a weighted average of all the positive differential values(negative differentials being ignored), or by performing an angular fitto the differentials of all the rod scintillators, or by any othersuitable analysis means for determining the azimuthal angle from the rodscintillator differentials.

The polar angle is the overall angle between the detector axis and thesource direction. Theoretically, polar angle is orthogonal to theazimuthal angle and does not depend on the azimuthal angle. In someembodiments, the detector may determine the polar angle using the panelscintillator detection rate in comparison to a function of the rodscintillator counting rates. The panel scintillator may have asymmetrical angular sensitivity distribution, whereas the rodscintillators may have a strongly antisymmetric angular sensitivity dueto the shield which blocks particles from one side only. As a result,the panel and rod scintillators can have opposite sensitivitydistributions, which enables a unique polar angle determination based onthe scintillator data alone. Thus, the source location may be determinedfrom data acquired at a single orientation and a single position of thedetector according to some embodiments.

To calculate the polar angle, the detector may calculate a ratio inwhich the numerator is a function related to the highest rodscintillator rates or differentials, and the denominator is equal to thepanel scintillator rate. The detector can then compare the ratio or itsinverse to a predetermined angular correlation function which candirectly indicate the polar angle of the source. Preferably, the polarangle calculation is arranged to be independent of the calculatedazimuthal angle, so that a single angular correlation function can beused to calculate polar angle regardless of the azimuthal angle.However, the detection sensitivity of the rod scintillators isintentionally a very strong function of the azimuthal angle. Tonormalize the rod scintillator response, the detector may calculate anumerator equal to the highest rod scintillator rate, plus a geometricalfactor times the second-highest rate, wherein the geometrical factor canaccount for the fact that the detection efficiencies of the rodscintillators are generally different for particles arriving atdifferent azimuthal angles. The geometrical factor may thereby correctfor the shape-dependent detection efficiency of each rod scintillator inthe presence of the shield. In some embodiments, the geometrical factormay be adjusted to equalize the response of the rod scintillators forazimuthal angles of zero degrees and 45 degrees. By symmetry, then, allthe other multiples of 45 degrees (such as 90, 135, etc) have the sameangular correlation function, and the intermediate angles such as 22.5degrees are very close. In embodiments, it is found that a geometricalfactor of (2π√2)⁻¹≈0.11 accomplishes this desired equalization, whichthereby provides that the resulting angular correlation function isindependent of the azimuthal angle to high precision. Therefore, usingthe corrected ratio with the geometrical factor, the polar angle may beobtained from the correlation function according to the panel and rodscintillator rates directly, throughout the 4π sphere, without the needfor azimuthal angle corrections or any other corrections according tosome embodiments.

The azimuthal and polar angles may be calculated using the followingequations. Equation 1 specifies one way to perform the angularinterpolation, where φ(source) is the calculated azimuthal angle of thesource, φ(i) is the positional angle of the i'th rod scintillator, Dm isthe maximum differential, φ(m) is the positional angle of the rodscintillator with the maximum differential, Di are the other positivedifferentials (other than the maximum one), and i ranges over all thepositive differential scintillators in the detector, other than Dm. ThusDi steps through the other rod scintillators that partially face thesource. The interpolated azimuthal angle φ(source) is then given byEquation 1:φ(source)=φ(m)+Σ[(φ(i)−φ(m))×Di/(Dm+Di)]  (1)

As an example, with four rod scintillators (N=4), in general two of thedifferentials are positive. Equation 1 starts with the highest positivedifferential Dm, and then interpolates between the two positivedifferentials to obtain the azimuthal angle. If the source is directlyfacing one of the rod scintillators, the azimuthal angle is nearly equalto the positional angle φ(m) of that maximal rod scintillator. If thesource is about half-way between two of the rod scintillator positions,then the highest and second-highest differentials are nearly equal, andthe interpolation arrives at an angle half-way between them.

The polar angle of the source, θ(source), may be found by calculating avalue V, which is equal to the highest rod scintillator rate, plus thegeometrical factor times the second-highest rod scintillator rate. Thatsum may then be divided by the panel scintillator rate F, therebyobtaining a ratio R. The ratio R, or its inverse, may then compared tothe predetermined angular correlation function. The polar angleθ(source) can then be found as the value of the angular correlationfunction that corresponds to the observed value of R. Equations 2 and 3show this explicitly.R=V/F=(Dm+g*Ds)/F  (2)

Here R is the ratio of the rod and panel scintillator rates, Dm is thelargest differential among the rod scintillators, Ds is thesecond-largest differential, F is the panel scintillator detection rate,and g is the geometrical factor.θ(source)=PACF(R)  (3)

In Equation 3, θ(source) is the polar angle of the source, and PACF(R)stands for the predetermined angular correlation function, which takesas input the value of R and provides as output the polar angle. If theangular correlation function comprises tabular values, interpolation orweighted fitting or other analysis may be applied to split the discretevalues of the correlation function according to the actual value of Robtained from the scintillator data. When used in this way, the angularcorrelation function of Equation 3 is a deterministic, monotonicfunction that reads out the polar angle directly according to the Rcalculated from Equation 2. In embodiments, the predetermined angularcorrelation function may be any data set that correlates the sourcepolar angle to the ratio R or its equivalent, such as a set of tabulardata, an analytic function, a computer program that converts R (or theraw scintillator data) to the polar angle, a graphical data set wherethe polar angle can be read off the graph, or any other data set thatyields the polar angle from the scintillator data or the ratio R. Inpreferable embodiments, the angular correlation function may bedetermined in advance by measuring the scintillator rates while a testsource is moved around the detector at different polar angles, or bysimulation using a program such as MCNP or GEANT or other particletrajectory simulation program, or other calibration means well known inthe art.

In some embodiments, the detector can calculate the source angles fromthe scintillator counting rates or the number of counts in eachscintillator during a particular time period. Alternatively, any dataassociated with particle detections can be used instead of the countingrates, such as anode currents or accumulated charge or any other signalrelated to the number of particles interacting with each scintillator ina time interval. The angle calculating method may include subtracting apredetermined normal background rate from each scintillator detectionrate, and dividing by a predetermined detection efficiency of eachscintillator. The method may also include rejecting any illegalcombinations, such as two rod scintillators counting at the same time.In some embodiments, backgrounds such as cosmic rays can be rejectedaccording to pulse height or pulse shape or other logic.

In some embodiments, the detector may include means for measuring theorientation of the detector axis relative to a fixed coordinate system,such as true north and horizontal. An example of means for determiningthe absolute orientation is an accelerometer and an electronic compass,which together measure the pitch and yaw of the detector relative to aground-based coordinate system. The detector may also include a GPS(global positioning system) receiver for determining the detectorposition, which would also enable triangulation using multiplemeasurements at different positions to further localize the source inthree dimensions.

In most applications, it is sufficient to simply report the azimuthaland polar angles of the source. In some applications, however, it may benecessary to rotate the detector until it points directly at the source.In these cases, the source angles may first be calculated from detectiondata acquired with the detector at an initial orientation, and then thedetector may be rotated according to the calculated source azimuthal andpolar angles. A second set of data can be acquired at the neworientation, and if necessary, the rotation can again be performed. Inmany cases, a single rotation may be sufficient to bring the detectoraxis into alignment with the source. In some embodiments, alignment canbe verified by noting when all of the rod scintillators count at aboutthe same rate, or that all of the rod scintillator differentials areapproximately zero, or that the calculated polar angle is less than apredetermined limit, for example.

In some embodiments, the thickness of the panel scintillator is acritical design parameter because the panel scintillator thickness maydirectly affect the angular correlation function. In some embodiments,the panel scintillator may also partially block or shadow the rodscintillators for particles arriving from the frontward direction. Thepanel scintillator may be thick enough to detect most, or substantiallyall, of orthogonally incident particles, while the rod scintillators maydetect particles that pass beside the panel scintillator or pass throughthe panel scintillator after being detected. Alternatively, the panelscintillator may be thin enough to avoid excessively shadowing the rodscintillators, but thick enough for sufficient detection efficiency andfor rapid determination of the polar angle. In some embodiments, thethickness may be selected so that the panel scintillator detects atleast 10%, and more preferably 20-50%, and in some cases 90%, of theparticles orthogonally incident on it. Preferably, at least 50% of theorthogonally incident particles can travel through the panelscintillator, either with scattering or without interacting at all, sothat the penetrating particle may be detected in one of the rodscintillators. For example, a gamma ray could Compton scatter in thepanel scintillator, and then the scattered gamma ray can continue inabout the same direction to be detected by one of the rod scintillators.Likewise, a neutron can be elastically scattered in the panelscintillator and then scatter again in the rod scintillators. Thus,according to some embodiments, there can be three classes of particleinteractions: (1) the particle can be detected in the panel scintillatorand stop there, (2) the particle can pass through the panel scintillatorwithout interacting, and then be detected in the rod scintillator, and(3) the particle can interact and be detected in the panel scintillator,and subsequently can interact and be detected again in the rodscintillator. When the same initial particle triggers both the panel androd scintillators, both counts are valid because they are both caused bythe same initial particle. In most cases and at most energies, thescattering angle is sufficiently small that the scattered particle isunlikely to cross over and trigger the “wrong” rod scintillator.

The panel scintillator design was tested using MCNP6. Simulated 1 MeVgamma rays were incident orthogonally on a panel scintillator comprisingBGO with a thickness of 10 mm. Any gamma rays that passed through thepanel scintillator were then detected in a large downstreamscintillator. There was no shield in this simple test because thepurpose was to quantify the panel scintillator performance. The resultswere as follows: 25% of the particles were detected in the panelscintillator and were fully stopped there, 65% of the particles passedthrough without interacting and then were detected in the downstreamscintillator, and 10% of the particles were detected in the panelscintillator and then were detected again in the downstreamscintillator. Stated in terms of detection probability, the panelscintillator had an overall detection probability of 35% while thedownstream scintillator had an overall detection probability of 75%.This adds up to more than 100% because the detection probabilityincludes particles that were detected by both the front and downstreamscintillators. The simulation also shows that the shadowing caused bythe panel scintillator is relatively small, just 25% of the incidentparticles. The conclusion of the simulation is that the panelscintillator thickness can be adjusted to provide sufficient detectionof incident gamma rays while still allowing over 50% of the particles topass through to the rod scintillators.

In some embodiments, the panel scintillator may be oriented orthogonalto the detector axis and positioned frontward of the rod scintillators.In a first embodiment, the panel scintillator spans the full width ofthe detector, or the full width of the rod scintillator array. Anadvantage of making the panel scintillator the same size and shape asthe rod scintillator array is that the resulting assembly may be compactwhile providing an adequate detection efficiency for the panelscintillator, and still allows a sufficient number of particles to reachto the rod scintillators. In a second embodiment, the panel scintillatormay be smaller in lateral width than the rod scintillators. An advantageof making the panel scintillator smaller than the rod scintillator arrayis that part of the rod scintillator area can be exposed directly to thesource, without attenuation by the panel scintillator. In a thirdembodiment, the panel scintillator extends laterally beyond the rodscintillators, thereby enhancing the panel scintillator detection rateand/or tailoring the shape of the angular correlation function.

In some embodiments, the panel scintillator may be a single monolithicslab-shaped scintillator body, which has the advantage of simplicity offabrication. A monolithic panel scintillator can also provide economysince only one light sensor may be needed for the panel scintillator.Alternatively, the panel scintillator may be partitioned into N separateportions, one portion per rod scintillator, with each such portion beingmounted frontward of one of the rod scintillators respectively. Forexample, if there are four rod scintillators, the panel scintillator canbe split into four separate portions, one portion being mountedfrontward and adjacent to each of the rod scintillators. Preferably,each panel scintillator portion may have a shape and size thatsubstantially matches the frontal surface of the corresponding rodscintillator. An advantage of partitioning the panel scintillator inthis way is to simplify construction, since each panel scintillatorportion may be attached to one of the rod scintillators, and then eachsuch assembly can be inserted into the shield structure. Partitioningthe panel scintillators may also be convenient when the shield protrudesfrontward beyond the rod scintillators, since the separate panelscintillator portions can be inserted in the space below the protrusiondistance. Although it may seem that partitioning the panel scintillatorwould ruin its required symmetrical angular sensitivity, this is easilyresolved in analysis. For example, the processor may simply add togetherthe data from all of the panel scintillator portions, therebyeffectively reassembling a net total panel scintillator signal, just asif the panel scintillator comprised a single monolithic slab. The totalpanel scintillator signal is highly symmetrical, thereby enabling thepolar angle calculation described above.

The panel scintillator portions may be viewed by a light sensor in atleast two ways. As a first option, each panel scintillator portion maybe optically isolated from all the other scintillators, in which caseeach portion can be viewed by a separate light sensor. The light sensorfor each single panel scintillator portion preferably can be a compactphotodiode sensor so that it would intercept as little of the incomingradiation as possible. Such an optically isolated panel scintillatorportion can have the advantage of simplicity since each portion producesa discrete signal according to some embodiments.

As a second option, each panel scintillator portion may be opticallycoupled to the adjacent rod scintillator, in which case a single lightsensor can view both the rod scintillator and its attached panelscintillator portion simultaneously. For example, each rod scintillatormay be connected to one of the panel scintillator portions on the frontend, and to a light sensor on the back end of the rod scintillator.Coupling the panel and rod scintillators in this way is economicalsince, according to some embodiments, only a single light sensor isneeded to detect light from both scintillators. Also, it allows placingthe light sensor at the back of the detector, thereby ensuring thatincoming particles from the front are not blocked by the light sensor.The panel scintillator, when optically coupled to the rod scintillator,can be configured to produce detectably different light pulses from therod scintillator, such as different pulse shapes, so that the processorcan determine whether the interaction was in the panel scintillatorportion or the rod scintillator (or both), by analyzing the pulse shape.

In some embodiments, the detector shield comprises N shield platesabutted or joined at the detector axis. Alternatively, and equivalently,the shield may comprise N/2 shield plates that mutually intersect at thedetector axis. The shield may be configured to isolate each rodscintillator from each other rod scintillator, or more specifically, toprevent a particle from passing out of one rod scintillator and intoanother rod scintillator. This function of the shield is in contrast toprior-art shields and collimators which are configured to prevent somesubset of the incoming particles from reaching the detector at all,which necessarily reduces the detection efficiency due to the lostparticles, and invariably increases the weight. The shield disclosedherein, on the other hand, can be light-weight, compact, and retain highefficiency since it can be surrounded by the scintillators according tovarious embodiments.

In some embodiments, each of the individual shield plates preferably canbe thick enough to block or attenuate most (over 50%) of the particlesincident orthogonally on the shield plate, but the effective isolationis much higher in practice because most of the particles incident oneach shield plate are not orthogonal; they arrive at random angles.Therefore, each particle, on average, has a tangentially longer pathlength in the shield plate according to its particular angle. Thiseffectively multiplies the amount of attenuation by a factor that rangeson average from 2 to 3, resulting in typically 80-90% isolation of therod scintillators, which in various embodiments, is more than sufficientfor the angular determination.

As used herein, a particle is blocked by a shield or shield plate if theamount of energy passing through the shield or shield plate is less thanthe amount of energy that a scintillator requires for detection. In someembodiments, the signal detection threshold is set at about 10% of theinitial particle energy, so if less than 10% of the initial particleenergy penetrates the shield or shield plate, the event will not bedetectable in a downstream scintillator. For example, in an applicationto detect 1 MeV gamma rays, the threshold could be set at 100 keV, inwhich case the shield or shield plate would be sufficient to block orattenuate most of the orthogonally incident particles if, for over halfthe orthogonally incident particles, the amount of energy passingthrough the shield is less than 100 keV.

In some embodiments, the shield may protrude frontward beyond the rodscintillators by a protrusion distance. The protrusion distance ispreferably sufficient to block particles arriving at oblique angles,such as 45 degrees, thereby preventing them from triggering thedownstream rod scintillator. In some embodiments, the protrusiondistance can be related to the lateral dimensions of the rodscintillators. For example, the shield protrusion distance may be about0.25 times the sum of the largest and smallest lateral dimensions of therod scintillator, or the protrusion distance may be proportional to aradial dimension of the rod scintillator relative to the detector axis.In addition, the protruding region of the shield may be shaped at thefront end of each shield plate, so as to tailor the angular response ofthe rod scintillators. The front surface of each shield plate may be cutsquare, or peaked, or rounded on its frontward surface for example. Theprotrusion may comprise the same material as the rest of the shield, ora different material. For example, a lead shield may be topped by atungsten protruding section to optimize the angular sensitivity of therod scintillators.

The detector may include a shield slug, comprising shielding materialdisconnected from the shield, and positioned frontward of the panelscintillator. In some embodiments, the shield slug is similar in shapeto the shield, but extends frontward of the panel scintillator. Theshield slug may be rectangular in cross-section, or peaked, or roundedat its frontward surface according to the angular sensitivity desired.The shield slug may comprise a different material from the shield, suchas a steel shield with an osmium slug. In some embodiments, the shieldslug material may be selected to have a higher shielding property thanthe shield material, such as a material with higher Z and higher densitythan the main shield.

In some embodiments, each rod scintillator may be a simple prism shapesuch as a square shape which is extruded in the third dimension(parallel to the detector axis). Optionally, the rod scintillators maybe reshaped or modified in various ways. For example, the rodscintillators may be beveled on their frontward surface to improve thepolar angle determination. In some embodiments, the bevel can be cut soas to remove scintillator material from the region of the rodscintillator which is farthest from the detector axis, since this regionis the least protected against incoming radiation by the shield. Forexample, if the rod scintillator is not beveled, any incoming particlesat an angle of 45 degrees to the detector axis may pass in front of theshield and then strike the downstream rod scintillator, thereby dilutingthe angular data. If the rod scintillators are beveled, however, theoblique particle would miss the downstream scintillator entirely. Thusthe beveling improves both the azimuthal and polar angle calculations byavoiding “contamination” of the detection data by particles that passover the shield and interact with the downstream rod scintillator.Preferably the bevel angle is large enough to substantially avoiddetecting particles that arrive from the opposite side of the detector,but not so large that the overall detection efficiency is substantiallyreduced. In some embodiments, the bevel angle may be 30 to 60 degreesrelative to the detector axis.

In some embodiments, the processor may be configured to determine whenthe detector axis is aligned or nearly aligned with the source bydetermining when all of the rod scintillators have substantially equalcounting rates. The detector may then activate an indicator, orotherwise report that the source is at the detector's aim point.Optionally, the indicator can be modulated according to the polar angle,for example by adjusting the indicator's intensity or flicker frequencyor other parameter according to the polar angle being large or small,and thereby indicate how close the detector axis is to the sourcedirection. If the shield protrudes somewhat beyond the scintillatorarray, angular precision of better than one degree can be obtained bythis “equal-rates” criterion, whereas without such a shield protrusionthe achievable angular resolution is generally poor, such as 5-10degrees in the prior art. Preferably the detector axis can be aimed atthe source with sufficiently low uncertainty that the source can belocalized among obfuscating clutter, which in most inspection situationsrequires an accuracy of about 1 degree.

Optionally, the processor may be configured to store information aboutthe detector orientation, the source location, the scintillator signals,the analysis results, and/or other information related to particledetection or source localization. Various embodiments may includewireless or wired or optical communication means for transmitting and/orreceiving information with another system, optionally includinginformation about the detector position and/or the detector orientationat the time when the angular results were obtained. Various embodimentsmay communicate with another similar detector in a local network, orwith a central computer that analyzes and archives data from multipledetectors.

In some embodiments, the detector may include a light beam emitter thatcan emit a light beam. In a first embodiment of the emitter, the lightbeam is directed along the detector axis, thereby indicating to anoperator where the detector is currently aimed. In addition, the beamshape may be configured as a wedge or arrow or other directional shape,and oriented so as to indicate the azimuthal angle of the sourcevisually, thereby helping the inspector find the source. Even morepreferably, the size or shape or other parameter of the beam spot may bechanged in a distinctive way depending on whether the polar angle islarge or small, so that the operator can tell visually how close the aimpoint is to the source location. When the detector axis has been broughtinto substantial alignment with the source, the processor may change theshape or other feature of the light beam to indicate that alignment.This can be accomplished, for example, by making the beam spot circularor as a cross-hairs when aligned with the source, by changing the colorof the beam when the detector axis is aligned with the source, bymodulating the beam intensity, or by some other variation of the beamspot so as to show the operator that the detector axis is substantiallyaligned with the source location. The beam spot may further indicate theuncertainty in the polar angle determination by, for example, beinglarger or smaller according to the uncertainty. The modulation can alsoprovide visual directional hints to the operator when the detector axisis close but not exactly aligned with the source, such as a circularbeam spot with an asymmetric intensity pattern, which may be modulatedtemporally to indicate “close but not yet right on.” Thus, the beam spotflicker modulation greatly assists the operator in locating the sourceprecisely.

In a second embodiment of the light beam emitter, the light beam is notdirected along the detector axis, but rather, is directed toward thecalculated source location. To redirect the light beam, the light beamemitter itself may be rotatable using a gimbal, or a movable mirror orsome other optical element may be used for redirecting the light beamtoward the source direction according to some embodiments. In suchcases, the light beam can be illuminated as soon as a source isdetected, and the beam could then be directed toward the sourceaccording to the calculated azimuthal and polar angles, therebyilluminating the source location with the light beam. Such a directedbeam can thereby provide a visually unmistakable indicator of the sourcelocation. The operator can then easily see where the source is located,namely at the spot illuminated by the beam. The beam spot may also beshaped, as an ellipse or rectangle for example, according to theuncertainties in the azimuthal and polar angle determinations, in whichcase the size of the beam spot may become progressively smaller as theangle determinations are improved with further detections. Since thedetector may be usable both indoors and outside, day and night, the beamintensity may be adjusted according to the ambient light to provideenough brightness for easy visibility. Also, the light beam may beflickered or otherwise modulated to further increase its visibility insunlight or among clutter. As the detector is moved or rotated, thepolar and azimuthal angles can be continuously recalculated, and thelight beam direction can be updated accordingly. The beam spot in thatcase can then seem to be “locked on” to the source location, remainingon that one spot no matter how the instrument is moved or rotated. Insome embodiments, a tone or other non-visual indicator may be activatedto alert the operator that a source has been found.

In some embodiments, the detector may include a camera such as a stillor video camera. For example, in a first embodiment, the camera may bealigned with the detector axis, thereby imaging the scene as viewed bythe detector. Preferably the current aim point of the detector is at thecenter of the image, or is otherwise indicated on the recorded imagesuch as with a cross-hairs superposed on the image. Optionally thecalculated source location may be indicated by an icon superposed on theimage. The size of the icon may be configured to indicate theuncertainty in the source location determination. The processor mayactivate the camera automatically when the processor calculates thesource location or when the detector axis is substantially aligned withthe source. According to various embodiments, the camera may beconfigured to collect images continuously, or only upon operatorcommand, or when the inspection begins and periodically thereafter, orwhenever the detector is rotated, or according to other criteria.Further data may be superposed on the image or otherwise saved alongwith the image data, such as the state of the scintillators, thecalculated azimuthal and polar angles, and/or other information relatedto the inspection.

In a second embodiment of the camera, the camera may be configured toacquire images centered not on the detector axis, but rather, on thecalculated source direction. Thus the image can be centered according tothe calculated azimuthal and polar angles, thereby recording the sourcelocation in detail. In some embodiments, the camera itself may berotatable, using a gimbal for example. Alternatively, the camera mayhave a movable mirror to redirect the view to the calculated sourcedirection. Preferably the source location is directly in the center ofthe image, and may be indicated by a superposed icon that may alsoindicate the uncertainty in the azimuthal and polar angle determinationsat the time the image was acquired. As a further advantage of centeringthe image on the source location, the camera could easily magnify orzoom in on the source location without having to adjust the aim point.For example, in some embodiments, the camera centered on the sourcelocation can automatically take a series of photos with differentmagnification, thereby automatically exposing successively greaterdetails about the source and its nearby clutter.

In some embodiments, the detector may be part of a hand-held meter withone or more haptic indicators configured to convey detection informationto an operator of the detector. The haptic indicators may comprisepulsed or vibrating pads attached to the detector handle. The hapticindicators may be activated to alert the operator that the radiationlevel has increased above a limit, optionally as determined from thescintillator rates or other data. In some embodiments, the hapticindicators may be pulsed when the detector axis is brought intoalignment with the source location, thereby informing the operator ofthe alignment without the operator having to look away from the scene.In some embodiments, a haptic indicator may be modulated according themagnitude of the polar angle, for example pulsing slowly when the polarangle is large and more rapidly as the aim point is brought closer tothe source location. In some embodiments, a plurality of hapticindicators may be mounted around the handle and activated individuallyto indicate the azimuthal angle. The operator would thereby know whichway to rotate the detector to approach the source location.

Turning now to the figures, FIG. 1 is a perspective sketch showing thespherical angles measured by a directional detector 100 according to thepresent disclosure. The detector axis, shown as a dot-dash line, is thecentral symmetry axis of the detector 100. The midplane 110 is shown asa dotted plane. The panel scintillator 103 is shown in stipple. Thesource is indicated by a star, and the angles by arrowed arcs. Theazimuthal angle is a rotational angle measured around the detector axis,going counter-clockwise, with zero degrees at the right-side horizontalplane (shown in dash) as viewed from behind the detector 100. The polarangle is the overall angle between the detector axis and the source. Thepolar and azimuthal angles thus localize the source in two dimensions.

FIG. 2 is a perspective sketch of an embodiment of the detector,comprising four rod scintillators 201, a shield 202 (edges shown indiagonal hatch), and a panel scintillator 203 (in light stipple). Asshown by this figure, each rod scintillator 201 may have a right squareprism shape, comprising scintillator material configured to produce alight pulse when traversed by an energetic charged particle such as anelectron or an ion. The panel scintillator 203 may be a slab, configuredto emit a light pulse when traversed by an energetic charged particle,and oriented perpendicular to the detector axis 208, and positionedfrontward of the rod scintillators 201, and centered on the detectoraxis 208. The shield 202 comprises four shield plates, of which only twoedges are visible in the sketch. The shield plates may be joined at thedetector axis 208 so as to form a four-bladed fan, symmetricallyarranged around the detector axis 208. Alternatively, and equivalently,the shield 202 may comprise two larger plates that intersect at themiddle of each plate, thereby forming the same structure as thatdepicted. The shield 202 may substantially abut the panel scintillator203 to maximize the detection efficiency in a compact assembly, or thepanel scintillator 203 may be spaced apart from the shield 202 and therod scintillators 201 for ease of assembly. The rod scintillators 201may all be perpendicular to the panel scintillator 203 as shown in thisfigure.

In operation, the embodiment of FIG. 2 determines the polar andazimuthal angles of a source (not shown) of particles such as gamma raysor neutrons. The particles interact in one of the rod scintillators 201or in the panel scintillator 203. The interaction produces secondariescomprising gamma-generated electrons or neutron-generated ions. Thesecondary particles then cause the scintillator 201 or 203 to emit alight pulse, which may be detected by a light sensor (not shown) andanalyzed by a processor (not shown). The processor may calculate theazimuthal angle by comparing the counting rates (or other detectiondata) in the various rod scintillators 201. For example, the processormay determine the azimuthal angle of the source by interpolating betweenthe angular positions of the two highest-counting rod scintillators 201.Or, more preferably, the processor can first calculate a differentialassociated with each rod scintillator 201, wherein the differential isequal to the counting rate of each rod scintillator 201 minus thecounting rate of the diametrically opposite rod scintillator 201. Theprocessor may then interpolate between the two highest differentials, tofind the azimuthal angle of the source.

In some embodiments, the processor may calculate the polar angle bydividing a function of the rod scintillator 201 detection rates by thepanel scintillator 203 detection rate, thereby obtaining a ratio. Theprocessor may then compare that ratio to a predetermined angularcorrelation function, which directly reveals the polar angle of thesource. In a preferred embodiment, the numerator equals the highestdifferential, plus a geometrical factor (usually 0.11) times thesecond-highest differential. With that formula, the same angularcorrelation function can be used for all azimuthal angles, which is asubstantial simplification. The polar and azimuthal angles so calculatedthereby locate the source direction relative to the detector axis 208 intwo dimensions. In some embodiments, the source location may bedetermined using only the scintillator data acquired at a singleposition and a single orientation of the detector, and with norotations, iterations, or complicated analysis required.

The embodiment of FIG. 2 is particularly efficient because the shield202 is almost entirely surrounded by active scintillators 201 and 203,with only the edges of the shield 202 plates exposed to incomingparticles. This is in contrast to prior-art systems in which many of theincident particles are lost by striking collimators or other externalshields which block the particles before they reach any scintillator. Invarious embodiments of the detector disclosed herein, the incidentparticles encounter one of the active scintillators 201 or 203 first,and the particle reaches the shield 202 only after passing through oneof the active scintillators 201 or 203. The shield 202 is not configuredto block particles from reaching the detector; it prevents particlesfrom passing between the rod scintillators 201. As a result, thedetector avoids losing useful particles and provides a surprisingly highdetection efficiency for a directional detector. In addition, thedetector can determine the polar and azimuthal angles of sourcesanywhere in the front half-space with high precision.

In an embodiment for detecting gamma rays, the rod scintillators 201 andthe panel scintillator 203 may be any organic or inorganic scintillator,but preferably not ZnS which is not sensitive to the lightly-ionizingelectrons from gamma interactions. If the scintillators 201 and 203 arehygroscopic such as NaI, each scintillator 201 and 203 can be enclosedin a sealed enclosure, or else the entire detector could be hermeticallysealed. In the embodiment of FIG. 2, as an example, the rodscintillators 201 may be PVT scintillators, with dimensions 35 mmsquare, and 150 mm long in the front-back direction. The panelscintillator 203 may be BGO with a thickness of 10 mm. The shield 202preferably comprises any high-Z, high-density solid such as lead ortungsten, although other materials such as leaded glass or steel canwork if sufficiently thick. The thickness of each plate of the shield202 can be thick enough to block or attenuate over 50% of theorthogonally incident gammas, thereby providing sufficient isolationbetween the rod scintillators 201 to enable the azimuthal angledetermination with just a few detected particles. Likewise, the polarangle can be determined from the panel scintillator 203 data with just afew detected particles according to the predetermined angularcorrelation function.

In some embodiments, the thickness of the panel scintillator 203 isthick enough to detect a sufficient number of the incident particles toprovide a definite polar angle determination, but not so thick that itsignificantly shadows the rod scintillators 201. In some embodiments,the thickness of the panel scintillator 203 is sufficient to detect 10%to 50% of the orthogonally incident particles, with the rest travelingthrough the panel scintillator unscattered. In other embodiments, thepanel scintillator 203 is thick enough to detect substantially all ofthe particles orthogonally incident on the panel scintillator 203, whilethe rod scintillators 201 primarily detect particles that pass besidethe panel scintillator 203 as well as particles that scatter in thepanel scintillator 203 and then pass through to the rod scintillators201.

In some embodiments, the design can be adapted for detecting fastneutrons. For example, the rod scintillators 201 and the panelscintillator 203 may comprise stilbene or a plastic scintillatormaterial that incorporates an ionization-density-dependent fluor thatproduces different pulse shapes when traversed by an electron versus arecoil proton. Alternatively, the scintillators 201 and 203 couldcomprise an inorganic scintillator that produces different pulses fordifferent particles, such as CsI, embedded in a transparent hydrogenousmatrix, such as acrylic. Such ionization-dependent scintillators enablediscrimination between neutron and gamma events on the basis of thepulse shape. Alternatively, the scintillators 201 and 203 could be ZnSembedded in a hydrogenous matrix such as acrylic and optionallyconfigured with a wavelength shifter for improved light collection. Insome embodiments, a shield 202 suitable for fast neutron shieldingcomprises a hydrogenous polymer such as HDPE, preferably made thickenough to degrade most of the fast neutrons incident on the shield 202by multiple elastic n-p scattering, so that the scattered neutrons haveinsufficient energy to be detected in the rod scintillators 201 afterscattering in the shield 202.

For detecting slow neutrons, the rod scintillators 201 may be ZnS,embedded in a transparent matrix with wavelength shifters, and loadedwith a neutron-capture nuclide such as ¹⁰B or ⁶Li, and configured sothat the emitted ions from neutron capture may readily pass into thescintillator. The shield 202 for slow neutrons may comprise HDPE loadedwith a neutron-capture nuclide such as LiF. Lithium is a preferredneutron-capture nuclide for shielding because it efficiently absorbslow-energy neutrons while generating very few gamma rays. Alternatively,if the scintillators 201 and 203 are of the gamma-blind type, then theshield 202 may include a neutron capture nuclide that has a largercapture cross section but emits gamma rays, such as B or Gd.

FIG. 3 is a rear-view perspective sketch of a detector similar to FIG. 2but with additional options. Four rod scintillators 301 may bepositioned in, and mutually isolated by, a shield 302 shaped as aplus-symbol (edges are shown in diagonal hatch). A panel scintillator303 may sit frontward of the rod scintillators 301 (only three areshown) and frontward of the shield 302. The detector as shown pointsaway from the viewer. In addition, light sensors 304 such as phototubesview the rod scintillators 301, and small light sensors 314 such asphotodiodes view the panel scintillator 303. The back light sensors 304are preferably positioned on the rear surfaces of the rod scintillators301 so as to avoid blocking particles arriving from the fronthalf-space. The small front light sensors 314 may be aligned with theshield 302 to minimize obscuring the rod scintillators 301 for particlesarriving from various angles.

Alternatively, instead of the small light sensors 314, a fifth phototube(not shown) can be mounted centrally on the panel scintillator 303. Thefifth phototube may block some incoming particles, but such blockingeffects can be fully accounted for in the angular correlation functionAlso, the fifth phototube, if centrally mounted, may be positionedmainly over the shield 302 area, thereby further minimizing anyinterference with the rod scintillators 301.

As a further alternative, instead of using separate phototube lightsensors 304, the rod scintillators 301 can be viewed by a large planarphotocathode, followed by a multichannel plate charge amplifier, allmounted across the back of the detector. The charge amplifier can beconfigured with four separate anodes to record light from each of therod scintillators 301 separately. Likewise, the panel scintillator 303can be viewed by a similar planar photocathode-multichannel amplifier,but with a single anode spanning the panel scintillator. Such planarlight sensors can be made thin enough to avoid blocking a significantfraction of the incoming particles according to some embodiments.

The panel scintillator 303 is shown extending laterally beyond the rodscintillators 301 by a distance 328. The additional area of the panelscintillator 303 can provide additional detection efficiency, and canalso tailor the angular correlation function in various ways dependingon the other design parameters.

The figure also shows the radial width 329 of the shield, which is thedistance from the outer edge of the shield 302 to the nearest surface ofthe adjacent shield plate. In some embodiments, the rod scintillators301 can fit into that space. Accordingly, the lateral width 330 of a rodscintillator 301 can be equal to the radial width 329 of the shield 302.

FIG. 4 is an exploded view of the detector with N=4. Accordingly, fourshield plates 402 can be assembled symmetrically abutting the detectoraxis 408. Or, equivalently, two larger plates (not shown) that span therod scintillator array 401 may be configured to intersect at the midlineof each of the larger plates, thereby comprising a structure equivalentto the four shield plates 402 shown. Four rod scintillators 401 (onlyone shown) may be inserted between and among the shield plates 402 sothat each shield plate 402 is adjacent to exactly two of the rodscintillators 401 and each rod scintillator 401 is adjacent to exactlytwo of the shield plates 402. Each rod scintillator 401 may comprise aright square prism with an extrusion axis parallel to the detector axis408 and perpendicular to the panel scintillator 403. The panelscintillator 403 may then be emplaced frontward of the rod scintillators401 and orthogonal to the detector axis 408 and centered on the detectoraxis 408 as indicated by arrows. The assembled detector faces theviewer.

FIG. 5 is a perspective sketch, partially exploded, of a detector with afour-sided shield 502, four rod scintillators 501 (only three shown), apanel scintillator 503, light sensors 504 attached to the rodscintillators 501, and small light sensors 514 attached to the panelscintillator 503. The detector is facing away from the viewer. Forgraphical clarity, only the edges of the shield 502 are shown indiagonal hatch, with all other surfaces shown unadorned. When assembled,a fourth rod scintillator 501 can fit into the empty slot of the shield502, and the panel scintillator 503 may then be mounted frontward of therod scintillators 501 either abutting the front surface of the shield502, or spaced apart from the front surface of the shield 502, forexample to accommodate a hermetic enclosure (not shown) or reflectivematerial (not shown) between the panel scintillator 503 and the rest ofthe detector.

The plates of the shield 502 are tapered in this embodiment, beingthicker in the front and thinner in the back. Tapering can reduce theweight. In some embodiments, a tapered shield 502 can provide sufficientisolation between the rod scintillators 501 if the maximum thickness ofeach shield 502 plate is sufficient to block or attenuate over half ofthe orthogonally incident particles. The rod scintillators 501 may beregular square-cross-section prisms, or they may be tapered oppositelyto the shield 502 so as to fit tightly into the shape of the openingbetween the tapered shield plates 502. As an added benefit, the partialtapering of the rod scintillators 501 can also enhance light collectionby reflecting scintillation light preferentially toward the back of thedetector.

FIG. 6 is a perspective sketch of a directional detector with rodscintillators 601, a shield 602, and four separate panel scintillatorportions 603. Each panel scintillator portion 603 can be mountedadjacent to the front surface of one of the rod scintillators 601,respectively. Also, the shield 602 may be truncated, or cut short at theback end, by a truncation distance 629 in order to save weight. Stateddifferently, the rod scintillators 601 can extend rearward beyond theback end of the shield 602 by the truncation distance 629. The shield602 can also protrude frontward beyond the rod scintillators 601 andoptionally beyond the panel scintillator portions 603, so as to provideimproved angular resolution. Also the protrusion may be peaked in thefrontward surface to further tailor the angular response. Each panelscintillator 603 can be optically coupled to its adjacent rodscintillator 601, and light sensors (not shown) can be mounted on theback surface of each rod scintillator 601. For example, the panelscintillator portions 603 can be made of BGO, while the rodscintillators 601 can be made of plastic PVT material, resulting inlight pulses with sufficiently different decay times (5 ns versus 300ns) so that they can be separated by pulse-shape analysis.

FIG. 7 is a perspective sketch, partially exploded, of an embodimentcomprising rod scintillators 701, a shield 702, a panel scintillator703, and a shield slug 712. Small light sensors 704 can be attached tothe corners of the panel scintillator 703. As an option, the rodscintillators 701 may be beveled 709 or 710, to tailor the angularresponse. The bevels 709 and 710 are shown cut at a bevel angle of about45 degrees relative to the detector axis (not shown, but centered on theshield 702 as usual). A partial bevel 709 is partial in that it coversonly a part of the frontward surface of a rod scintillator 701, while afull bevel 710 covers the entire frontward surface. In both cases, thebevel 709 or 710 can be configured to eliminate scintillator materialthat would be impacted by particles arriving at oblique angles, such as45 degrees, that pass over the front of the shield 702. Such particleswould otherwise be detected in the downstream rod scintillator 701,which would dilute the angular determination. The shield 702 is shownprotruding in the frontward direction, so as to provide high angularprecision in determining the polar angle of the source.

The shield 702 as depicted also slightly protrudes in the lateraldirections as well, to further isolate the various rod scintillators 701and enhance the determination of the azimuthal angle. The shield slug712 can be shaped much like the shield 702 in cross-section, therebyfurther sharpening the angular resolution by preventing particles frompassing over the shield 702 and striking one of the downstream rodscintillators 701.

FIG. 8 is a perspective sketch, partially exploded, of a detector withrod scintillators 801 in a 4-plate shield 802, and a novel panelscintillator 803 shaped to resemble the shape of the shield 802. Thusthe panel scintillator 803 is equivalent to a slab with four cornerpieces removed, so that the remaining volume has the samecross-sectional shape as the shield 802. Small light sensors 804 can bemounted on the ends of the “arms” of the shaped panel scintillator 803.An advantage of shaping the panel scintillator 803 in this way is thatthe rod scintillators 801 can be entirely unobstructed for sourceparticles when the detector is aimed at the source. Thus in someembodiments, there is no shadowing by the panel scintillator 803 forparticle from the front, since the panel scintillator 803 resides onlyover the shield 802. A second advantage is that the panel scintillator803 can act as a partial shield slug, similar to the shield slug 712 ofFIG. 7, thereby partially blocking oblique particles from reaching thedownstream rod scintillators 801 and thereby sharpening the angularresolution.

FIG. 9 is a partially exploded perspective sketch of a detector withfour triangular-prism-shaped rod scintillators 901, an “X”-shaped shield902, and a monolithic panel scintillator 903 that is intended to resideupon the front edge of the shield 902. In another embodiment, themonolithic panel scintillator 903 is mounted spaced above the front edgeof the shield 902, for example to leave room for light sensors (notshown). As an alternative, instead of the monolithic panel scintillator903, the detector may include four separate panel scintillators 913 asshown attached to one of the rod scintillators 901. In comparison to theother designs, the triangular rod scintillators 901 can have betterisolation and better angular resolution than a square cross-section rodscintillator, but lower detection efficiency due to the reduced volumeof scintillator. Also, the X-shaped shield 902 is 40% heavier than aplus-symbol-shaped shield of the same size and performance, such as 802in FIG. 8.

FIG. 10 is an exploded perspective sketch of a detector as acylindrical, four-sided detector. It is facing the viewer. Theconfiguration includes four rod scintillators 1001 (only three shown),each shaped as a 90-degree pie-section prism, and mounted into afour-plate shield 1002. A disk-shaped panel scintillator 1003 includesfour solid-state light sensors 1004. The light sensors 1004 may bepositioned in alignment with the shield 1002 plates, so as to minimizeshadowing of the rod scintillators 1001.

The pie-section-shaped rod scintillators 1001 have efficiencies andangular resolutions which are intermediate between those of the squarerod scintillators 801 of FIG. 8, and the triangular rod scintillators901 of FIG. 9. In some applications, the cylindrical shape of theembodiment of FIG. 10 can fit geometrical constraints better than theother shapes.

FIG. 11A is a perspective sketch of a bi-directional or double-endeddetector, with rod scintillators 1101, a shield 1102, a first panelscintillator 1103 and a second panel scintillator 1113. Light sensors1104 are also shown. The first and second panel scintillators 1103 and1113 can receive particles from the front and back half-space regionsrespectively, thereby covering a full 4π solid angle of view. Theprocessor (not shown) can determine whether the source is in front orbehind the detector by comparing the detection rates of the first andsecond panel scintillators 1103 and 1113. Then the processor can analyzethe rod scintillator 1101 detection rates to determine the azimuthalangle of the source. The processor may then calculate a ratio accordingto the rod scintillator 1101 data, divided by the counting rate ofwhichever panel scintillator 1103 or 1113 has the higher detection rate.The processor may then compare that ratio to a predetermined angularcorrelation function that directly indicates the polar angle of thesource.

The bi-directional or double-ended version of FIG. 11A is particularlyuseful in a mobile area scanner that searches for hidden weapon materialthroughout a wide area on both sides of the mobile scanner. Thebi-directional detector is also useful in a vehicle inspection stationin which multiple parallel lanes are active at the same time, so thattwo vehicles in adjacent lanes could be scanned simultaneously by adouble-ended detector stationed between the vehicles.

FIG. 11B is a perspective sketch of a bi-directional detector such asthat of FIG. 11A, but here the first and second panel scintillators 1123and 1133 are divided into portions, and each such portion 1123 or 1133is optically coupled to the intervening rod scintillator 1121, and allthree are viewed by a shared light sensor 1124. The shield 1122 isshown. Foil or other light reflective separators 1125 are disposedbetween the various first panel scintillator portions 1123 where theytouch, and likewise between the second panel scintillator portions 1133where they touch. The first and second panel scintillator portions 1123and 1133, and the rod scintillators 1121, are made from three differenttypes of scintillator material with distinct pulse shapes so that theycan be distinguished in analysis. For example, the rod scintillators1121 may be PVT with a 5 ns pulse width, the first panel scintillatorportions 1123 may be BGO with a 300 ns pulse width or NaI with a 230 nspulse width, and the second panel scintillator portions 1133 may be CaF₂with a 900 ns pulse width or CdWO₄ with a 1100 ns pulse width. Anadvantage of optically coupling the scintillators is economy, since onlyfour light sensors 1124 are needed in the embodiment shown. It may benoted that some particles are blocked by the light sensors 1124,resulting in an asymmetry, but this is easily resolved by calibration.

FIG. 12A is a longitudinal cross-section (that is, cut parallel to thedetector axis) sketch of an embodiment of the invention in which atransparent shield portion 1212 (shown in cross-hatch) can be opticallycoupled to a panel scintillator 1203 and to a light sensor 1204. Fourrod scintillators 1201 (two are shown) are viewed by other light sensors1214. The transparent shield portion 1212 serves as a light guide forlight pulses from the panel scintillator 1203. One advantage of thisconfiguration is that all of the light sensors 1204 and 1214 can bemounted on the back of the detector, away from incoming particles. As anexample, a detector configured for detecting fast neutrons could usetransparent acrylic or other transparent hydrogenous polymer for thetransparent shield portion 1212, which can thereby serve as both ashield and a light guide. For gamma ray applications, the transparentshield portion 1212 can be leaded glass with sufficient loading andthickness to block over 50% of gammas. For slow neutrons, thetransparent shield portion 1212 may be acrylic with LiF or other neutroncapture nuclide embedded in or coated on the acrylic.

The panel scintillator 1203 is shown smaller than the array of rodscintillators 1201. In these embodiments, the reduced-size panelscintillator 1203 improves the ability of the rod scintillators 1201 todetect particles when the detector axis 1208 is aligned with a source byleaving part of each rod scintillator 1201 unobscured from the front.Hence the panel scintillator 1203 can be made thick enough to detectsubstantially all of the particles orthogonally incident on the panelscintillator 1203 without substantially obscuring the rod scintillators1201. The rod scintillators 1201 are also shown with a front-end bevel1207 cut at an angle 1209 of about 45 degrees relative to the detectoraxis 1208. The bevel 1207 can improve the angular contrast by preventingparticles that pass over the shield portion 1212 from being detected inthe downstream rod scintillator 1201.

FIG. 12B is a transverse cross-section (perpendicular to the detectoraxis) sketch of the configuration of FIG. 12A with corresponding itemsbeing labeled the same. The four rod scintillators 1201 may be arcuatecross-section prisms separated by shield plates 1202 abutted to acentral transparent shield portion 1212 which may be cylindrical. Theshield plates 1202 are considered to substantially abut each other sincethey abut the transparent shield portion 1212, thereby forming asubstantially continuous shield structure that isolates the rodscintillators 1201 from each other. The panel scintillator 1203, showndisplaced in this view, can be installed at the dashed circle 1213 andoptically coupled to the transparent shield portion 1212 which iscoupled to a rear-mounted light sensor 1204 (not shown in this view).When the detector axis 1208 is aligned with the source, the rodscintillators 1201 are almost entirely exposed to the source particles,due to the smaller size of the panel scintillator 1203. Also, most ofthe area covered by the panel scintillator 1203 can be occupied by theshield plates 1202 or the central transparent shield portion 1212.Therefore, the amount of shadowing over the rod scintillators 1201 isnegligible according to various embodiments.

FIG. 13 is an exploded perspective sketch of a directional detector withthree-fold symmetry, having three pie-sector-shaped rod scintillators1301, a first panel scintillator 1303 and a second panel scintillator1313 covering the front and back of the detector respectively, and lightsensors 1304. The panel scintillators 1303 and 1313 are thick enough inthis embodiment to detect substantially all of the orthogonally incidentparticles. The extrusion dimension of the rod scintillators 1301 is theshortest dimension of the rod scintillators 1301 in the depictedembodiment. The rod scintillators 1301 are mainly sensitive to particlesfrom the side, while the panel scintillators 1303 and 1313 are mainlysensitive to particles from the front or back. The contrastingsensitivity distributions of the rod scintillators 1301 and the panelscintillators 1303 and 1313 enable the polar angle to be determined bycomparing detection rates in the rod scintillators 1301 with the panelscintillators 1303 or 1313. In this embodiment, the shield 1302comprises a scintillator (shown in grid-hatch) configured to detectparticles as well as blocking the particles. For gamma rays, thescintillating shield 1302 may be NaI(Tl) or a higher densityscintillator such as LuAP. Light sensors 1304 can be connected to therod scintillators 1301, the scintillating shield 1302, and to the firstand second panel scintillators 1303 and 1313. The configuration enablessimultaneous determination of the source location in all directions and,based on the gamma energies measured in the scintillating shield 1302,the isotopic content of the source.

FIG. 14 is a transverse cross-section view of a hexagonal embodiment ofthe detector with N=6. It can have six triangular rod scintillators 1401(only one showing) with a star-shaped shield 1402 and separate panelscintillator portions 1403. A small photodiode light sensor 1404 mayview each panel scintillator portion 1403. Other light sensors (notshown) may view the rod scintillators 1401 from the back. As analternative, instead of the separate panel scintillators 1403, thedetector can have a monolithic panel scintillator 1413 with a centrallyplaced light sensor 1414 that would block very little of the incidentradiation due to its position over the central part of the shield 1402.The shield 1402 may comprise six shield plates abutted at the detectoraxis, or equivalently, three larger shield plates that span the width ofthe detector and intersect each other at the midline of each of thelarger shield plates.

FIG. 15 is a transverse cross-section sketch of a detector withrectangular symmetry. Rectangular rod scintillators 1501 may be mountedin a rectangular shield 1502. Each rod scintillator 1501 (only onevisible) may be adjacent to a separate rectangular panel scintillator1503 (three shown). The rod scintillators 1501 may be longer in the Xdirection than the Y direction, and this shape can affect the angularresolution of the azimuthal and polar angle determinations. The angularresolution depends on the lateral dimensions of the rod scintillators1501, such that a smaller rod scintillator dimension results in betterangular resolution, all else being equal. The detection efficiency, onthe other hand, is generally larger when the lateral dimensions arelarger. Consequently, the configuration of FIG. 15 can have betterangular resolution for measuring the vertical source angle than formeasuring the horizontal angle, but higher detection efficiency forparticles arriving from the top or bottom than from the sides. If thedetector were rotated 90 degrees around a horizontal axis such as thedetector axis (not shown), the detector can then obtain betterresolution horizontally and better detection efficiency for particlesarriving from the left and right sides.

Since the angular correlation function generally depends at least inpart upon the shape of the rod scintillator 1501, it may be necessary toprepare two separate angular correlation functions for the configurationof FIG. 15, with one correlation function for determining the horizontalcomponent of the polar angle, and a second function for the verticalcomponent. The geometrical correction factor g, introduced in Eq. 2,would likely not be sufficient to account for the substantialdifferences in horizontal and vertical properties of the configurationshown; hence the need for two separate angular correlation functiondeterminations. Or, in a worst-case situation, the angular correlationfunction could vary so much with the azimuthal angle, that a largenumber of correlation functions would be needed. In that case, theangular correlation may be rendered as a matrix of values which can thenbe interpolated in both azimuthal and polar angles to evaluate eachparticular source location. Alternatively, a global functional form maybe developed that takes as input the azimuthal angle calculated earlier,plus the panel scintillator rates and some function of the rodscintillator rates or differentials, and then produces as output thebest-fit polar angle determination. Such a function may be implementedas a computer program that automatically exploits the various two-foldsymmetries of the design and interpolates values when a functional fitcannot be derived.

FIG. 16 is a flowchart showing steps of a method for calculating theazimuthal and polar angles from the scintillator data. This methodassumes a symmetrical configuration such as that of FIG. 2, but otherconfigurations may be used according to various embodiments. First, at1601, scintillator signals can be acquired at a particular orientationof the detector, including rod scintillator and panel scintillatordetection data. After accumulating detection data for a predeterminedintegration time, or at any time, analysis proceeds by calculating 1602a differential for each rod scintillator, equal to the difference indetection rate of each rod scintillator minus the detection rate of thediametrically opposite rod scintillator. If N is odd, then each rodscintillator can have two nearly-opposing rod scintillators, and thedifferential is equal to each rod scintillator detection rate minus theaverage of the two nearly-opposing rod scintillators. Then the azimuthalangle can be calculated 1603 by interpolating between the two rodscintillators with the highest differentials. The azimuthal angle of thesource may thus be a weighted average of the position angles of the rodscintillators according to their associated differentials. If there aremore than two positive differentials (such as for N=6 which has threepositive differentials), then all of the positive differentials may beincluded proportionally in the azimuthal angle determination. As afurther alternative, instead of interpolating between differentials, theazimuthal angle may be calculated directly from the rod scintillatorrates, for example, by fitting them to a source location model or otherformula that relates the source angle to the rod scintillator rates. Thesource model can predict the scintillator detection rates as a functionof the azimuthal angle of the source. As an option for obtaining theangular results rapidly, the source model may be used to calculatevarious values of the azimuthal angle versus detection ratios in advanceand store them in computer-readable non-transient media. Then when theactual detection data is acquired, the previously-calculated values canbe used directly or interpolated for best resolution, therebydetermining the azimuthal angle.

The polar angle may then be determined 1604 by calculating a rodscintillator value V, which in this embodiment is equal to the highestdifferential plus a geometrical factor g times the second-highestdifferential. Typically, the geometrical factor g is about 0.11, butartisans can adjust g according to their specific designs, using asimulation program or with a test source. For example, g may be adjustedso that the ratio R is the same for a source azimuthal angle of zerodegrees and 45 degrees.

A ratio R may then be calculated 1605 by dividing V by the panelscintillator counting rate. The inverse ratio, 1/R, could be usedinstead of R, but the inverse can become mathematically problematic whenthe detector axis is brought into near-alignment with the source,because then the rod scintillators have nearly zero differential whichmakes 1/R become infinite. Likewise, if the panel scintillator showszero counts when the integration time expires, then additional datashould be taken before the polar angle can be calculated. Insufficientdata after a particular integration time may be an indication that thesource is weak or far away, or that there is no source at all.

Then, at 1606, the polar angle may be determined by comparing the ratioR to the predetermined angular correlation function, which can indicatethe polar angle directly. If the detector is symmetrical around thedetector axis, and the geometrical factor has been applied as specifiedabove, then a single angular correlation function is applicable for allazimuthal angles. Then 1607 the final polar and azimuthal angles can berecorded, displayed, transmitted to another system, or otherwisereported as needed for each application.

FIG. 17 is a flowchart showing steps of a method for rotating thedetector until the detector axis is aligned with the source. First,detection data can be acquired 1701 from the panel and rod scintillatorsfor a predetermined integration time, or for an adaptive interval, orsome other time interval. The rod scintillator rates may then becompared 1702 to normal background rates to determine if a source ispresent. Preferably, the scintillator detection rates may first becorrected for normal backgrounds and for the pre-calibrated detectionefficiency of each scintillator. A source is then likely present ifeither: (1) any of the scintillators exhibits a positive residualcounting rate significantly above the expected background rate, or (2)the sum of all the scintillators exhibits a statistically significantdetection excess. If the rates are not above backgrounds, the detectorthen can indicate 1703 that no source is detected. But if the rates areabove background, then the rod scintillators can be compared 1704 witheach other to determine if they are all substantially equal. Suchequality need not be exact; preferably the detector checks whether therod scintillator rates are equal to within the statistical uncertaintyof each rod scintillator data. Alternatively, the degree of alignmentwith the source may be evaluated by comparing the observed inequalitiesamong the various rod scintillators with a predetermined function thatcorresponds to a small polar angle, such as 1 degree. In either case, ifthe rod scintillator rates are equal according to the selectedcriterion, then the task is done 1705 and the detector axis is alignedwith the source.

If, however, the rod scintillator rates are above background and are notall equal, then the detector can calculate 1706 the azimuthal and polarangles of the source, using for example the method of FIG. 16. Thedetector may then be rotated 1707 according to the calculated azimuthaland polar angles, thereby bringing the detector axis into alignment withthe calculated source location. Additional scintillator data are thenacquired 1708 at the new orientation, and after the integration time orafter some amount of data is accumulated, the method can then loop backto step 1704, determining if all the rod scintillators havesubstantially equal rates. The iteration proceeds until the rodscintillators rates are all equal according to the selected criterion.Usually one rotation, or at most two rotations, is sufficient to alignthe detector axis with the source in two dimensions. In contrast,prior-art detectors require extensive iterative searching and multiplerotations to locate the source in one dimension. The method exploits theadditional information of the polar angle and thereby rapidly closesupon the source direction.

FIG. 18 is a graph showing the results of an MCNP6 simulation of adirectional detector configured in this case to detect gamma rays. Inthe simulation, the source was moved in the horizontal plane around thesimulated detector, with horizontal angles ranging from −90 degrees to+90 degrees relative to the detector axis, and with an elevation angleof zero. In spherical coordinates, this corresponds to a polar angleranging from zero to 90 degrees, and the azimuthal angle being eitherzero or 180 degrees depending on which side of the detector the sourceis on. The graph shows the counting rates, in arbitrary units, of thefour rod scintillators according to the numbering shown in the insetdrawing. The counting rates in rod scintillators 1 and 4 are shown asthe lines with “o” markings, having high counting rates when the sourceis on the left side (as expected). Rod scintillators 2 and 3, shown asthe lines with “x” markings, have high rates when the source is on theright. The dashed curve shows the panel scintillator counting rate,which is maximum when the source is in front of the detector.

The two rod scintillators numbered 2 and 3 have nearly the same countingrate since they are equally exposed to the source when the azimuthalangle is zero or 180 degrees. Likewise, the two rod scintillators 1 and4 show statistically the same response for the same reason. It isapparent from the graph that the rod scintillators exhibit anantisymmetric angular sensitivity relative to the detector axis, whilethe panel scintillator has a symmetric angular sensitivity. Thedifference in rates among the rod scintillators indicates that theazimuthal angle can be found from the rod scintillator data. Inaddition, the difference in symmetry between the panel and rodscintillators implies that a correlation function can be found thatrelates the polar angle to the panel scintillator rates. Both theazimuthal and polar angular results draw on the same set of detectiondata, obtained at a single detector orientation. Rotations and iterationare not necessary unless the application calls for the detector to befinally rotated into the source direction, in which case a singlerotation is usually sufficient.

FIG. 19 is a graph showing the angular correlation function derived fromthe data of FIG. 18. The graph shows how the polar angle is related tothe ratio R, which equals the highest rod scintillator differential plusg times the second-highest differential, all divided by the panelscintillator detection rate. The correlation is smooth, monotonic, andapproximately linear, with small non-linearities due to the edges of therod scintillators and to shadowing by the panel scintillator. Thecorrelation function can be modified by adjusting the size and thicknessof the panel scintillator, the length of the rod scintillators, and theshield properties such as truncation and protrusion. For example,extending the panel scintillator laterally beyond the rod scintillatorscan increase the panel scintillator counting rate while slightlyreducing the rod scintillator counting rates over a range of angles,which would tend to raise the correlation function value over thatrange, which could partially cancel the nonlinearities. However, suchfine-tuning is usually unnecessary since the correlation yields a uniquepolar angle for each value of R, and since the method provides ahigh-resolution determination of the polar angle throughout the fronthalf-space, from zero to 90 degrees polar angles, which particularlycovers the midplane region that is problematic for prior-art directionaldetectors that lack a panel scintillator. And, with the addition of asecond panel scintillator as discussed with FIG. 11, the polar angledetermination can be extended to a full 180 degrees, thereby coveringall directions of space (an entire 4π solid angle), and thereby enablingprecise determination of the source location everywhere around thedetector using a single set of scintillator data.

A second simulation was performed to test whether the correlation ofFIG. 19 is indeed applicable for all azimuthal angles as required. To doso, the simulation of FIG. 18 was repeated but with a source azimuthalangle of 45 degrees rather than zero degrees, so that the sourcelocation scanned from the upper right quadrant to the lower leftquadrant of the inset in FIG. 18. The results of the second run, tiltedat 45 degrees, were statistically identical to the first run, at zerodegrees, when analyzed as specified above. This agreement demonstratesthat a single universal angular correlation function can be used to findthe polar angle, regardless of the azimuthal angle of the source, fromthe axis to the midplane.

The correlation function depends on the size and shape and compositionof the rod scintillators, the panel scintillator thickness, and othergeometrical features of the detector. Therefore, it is recommended thatartisans recalibrate the angular correlation function for theirparticular detector parameters using a simulation program or by adetermination of R versus polar angle, methods of which are well knownin the art.

To demonstrate how the detector can find the azimuthal and polar anglesof the source with a single set of detector data, a worked example isnow provided. First the analysis will be performed graphically forvisualization, and then repeated numerically for optimal precision. Inthe simulation of FIG. 18, a gamma ray source was positioned at 60.0degrees horizontal angle and zero degrees vertical angle (correspondingto azimuthal=0, polar=60). Based on the detection data, the ratio R wascalculated according to Equations 2 and 3. As shown by a vertical dashedline at about R=1.22 on the chart, the correlation implies that thepolar angle is given by the horizontal dashed line which corresponds toabout 59 degrees, which is only 1 degree from the correct value.

An improved way of calculating the angles is by interpolation. Table 1shows how the source location can be derived from the data directly. Thepositional angles of the rod scintillators are listed, along with thesimulated detection rates of the scintillators:

TABLE 1 (Source at azimuthal = 0, polar = 60 degrees) Scintillatordetection rate (arb. units) position angle Rod scintillator 1: 0.182+135 degrees Rod scintillator 2: 0.922  +45 degrees Rod scintillator 3:0.905  −45 degrees Rod scintillator 4: 0.177 −135 degrees Panelscintillator: 0.674 —

First the azimuthal angle may be calculated from the rod scintillatorpositive differentials and the positional angles of the rodscintillators, as shown in Table 2:

TABLE 2 (positive differentials only) Rod scintillators positional angledifferential side 2 minus side 4 +45 degrees 0.745 side 3 minus side 1−45 degrees 0.723

The other differentials are negative and are ignored. The azimuthalangle can then be calculated by linear interpolation between thepositional angles using the differentials, this time using the formulaof Equation 4:φ(source)=(φ(2)*D2+φ(3)*D3)/(D2+D3)  (4)

Here φ(2) is the positional angle of rod scintillator number 2(top-right scintillator) and D2 is its differential, equal to thedetection rate of rod scintillator 2 minus that of rod scintillator 4.Similarly φ(3) and D3 represent the corresponding values for rodscintillator 3 (bottom-right). Applying the numbers of Table 2 toEquation 4, the resulting calculated azimuthal angle is φ(calc)=0.7degrees, which is close to the actual azimuthal angle of φ(source)=0.0degrees.

The polar angle can then be found by calculating the ratio R accordingto Equation 2. The value V equals the largest differential, plus 0.11times the second-largest differential, which for this case is V=0.825.The ratio R is then found, from Table 1, as R=V/F=1.223. The polar anglemay then be determined by interpolating between the two values of thecorrelation of FIG. 19 that span the value of R(0). From the MCNP6simulation, those values are: R(50)=1.048 at 50 degrees polar angle, andR(60)=1.234 at 60 degrees polar angle. Using these values for a linearinterpolation on the observed ratio of R=1.223, the calculated polarangle for the panel scintillator data is calculated to be θ(calc)=59.4degrees, which is again close to the actual polar angle ofθ(source)=60.0 degrees.

This example shows that the system as disclosed and the correspondingmethod can determine the azimuthal and polar angles from scintillatorcounting rates with sub-one-degree resolution, using just a singleacquisition and with no rotations or iteration.

Various embodiments of the directional detector disclosed herein enableadvanced radiation-detection applications, some of which are nowdescribed. FIG. 20 is a perspective sketch, partly hidden, of ahand-held survey meter that can determine the direction of the source intwo dimensions, and then display the azimuthal and polar angles of thesource in near real-time. In some embodiments, an operator can move themeter at will, while watching the source direction indicator, andthereby locate the source. The system can continuously recalculate theazimuthal and polar angles of the source and continuously update thedirectional information. In some embodiments, the meter shows thelateral direction toward the source (azimuthal angle), and how far thesource is laterally from the current orientation (polar angle). Themeter can also indicate when the detector axis is aligned with thesource, as determined by the rod scintillator rates being equal. Thus,the azimuthal indicator can tell the operator what direction to rotatethe detector, the polar indicator can tell the operator how far torotate it, and the alignment indicator can tell the operator when tostop.

The system of FIG. 20 may include a directional particle detector 2000(hidden lines in dash), which is contained in a case 2001 with a handle2002 and a display 2003. Typically, the operator holds the meter at someorientation, waits for a brief integration time, and then reads thedisplay 2003 which shows the calculated azimuthal and polar angles ofthe source. The operator can then rotate the meter in the azimuthaldirection as indicated in the display 2003, rotating more quickly if thepolar angle is large and more slowly if the polar angle is small, andstopping when the display indicates that the system is aimed directly atthe source. Thus the meter enables rapid convergence to a sourcelocation by determining both the azimuthal and polar angles of thesource in real time, and displaying those results visually.

The meter may also have a light emitter 2004 that can emit a light beam2005 configured to indicate the azimuthal and polar angles visually aswell. In a first version, the light beam 2005 is aligned with thedetector axis, and the beam spot (that is, the light beam intensitydistribution) is shaped to indicate both the azimuthal and polar anglesof the source. For example, the beam spot can appear as an arrow orwedge shape, configured to point in the azimuthal direction, and mayhave a length proportional to the polar angle. Such a visual indicatorof the source direction can greatly assist the operator in locating thesource quickly.

In a second version of the system of FIG. 20, the light beam 2005 can beredirected toward the calculated source position. For example, arotatable mirror (not shown) may reflect the light beam 2005 to pointdirectly at the source based on the azimuthal and polar angles ascalculated. Alternatively, the beam emitter 2004 itself can be rotatableand aimed according to the calculated azimuthal and polar source angles.The beam spot can be shined right on the source location. The light beam2005 may also be flickered or otherwise modulated to further enhancevisibility. The beam spot location can be updated continuously as themeter is moved around so that the beam spot appears to be “locked on” tothe source location as long as the operator does not move the meter toofast. And if the operator does rotate the meter too fast for the systemto follow, the directional detector 2000 can soon recalculate thecorrect angles based on further scintillator data and then redirect thebeam 2005 back to the source, further assisting the operator in locatingthe source. Also, the beam 2005 may be configured to indicate theuncertainties in the angular determination. For example, the shape ofthe beam spot may be configured as an ellipse with axes sized accordingto the uncertainties in the polar and azimuthal angle determinations.Then as additional data is acquired, and the angular uncertainties arereduced, the beam 2005 shape accordingly shrinks to a focused spot.

As a further option, the system of FIG. 20 may include a camera 2007which again has at least two versions. In a first camera version, thecamera 2007 can be aimed frontward in alignment with the detector axis,thereby recording the inspection scene frontward of the detector. In asecond version, the camera 2007 can be redirected (using a movablemirror, for example) so as to record a scene with the source centered inthe image. An advantage of the latter version, with the image centeredon the source location, is that the camera 2007 can then easily magnifyor “zoom in” to the source location, without having to adjust the aimpoint as the magnification changes. For example, the camera 2007 can beactivated or triggered as soon as the azimuthal and polar angles areinitially determined, and the camera 2007 or its viewpoint can beredirected so as to view the calculated source location centrally in theimage. The camera 2007 can then acquire a wide-angle image, anarrow-angle image, and a telescopic magnification image centered on thesource location, all in rapid succession. The sequence of images atdifferent magnifications can thereby fully record the source locationdespite clutter and shielding. In addition, a rectangle or ellipse orother icon can be superposed on the image indicating the calculatedsource location. The icon dimensions may be configured to correspond tothe horizontal and vertical angle uncertainties in the source locationas calculated at the time the image was acquired. Then, as the sourcelocation determination is improved with further data, the size of theindicator icon is correspondingly reduced in subsequent images.

As a further option, the meter of FIG. 20 may have four hapticindicators 2008 (two showing) mounted on the handle 2002. The hapticindicators 2008 may be activated according to the calculated azimuthalangle, thereby indicating to the operator in what direction the sourceis located (for example, the right-side haptic indicator 2008 may beactivated if the source is to the right). The haptic indicators 2008 mayalso be modulated so as to indicate the magnitude of the polar angle(for example, being modulated faster if the polar angle is large, andmore slowly if the polar angle is small, or vice-versa). Also, when thedetector axis is brought into alignment with the source, the hapticindicators 2008 may be activated in a characteristic way, such as allfour haptic indicators 2008 being pulsed at once, or in a circularsequence, or otherwise indicate that the detector axis is aligned withthe source. This would inform the operator of the source direction andthe source alignment without the operator having to look away from thescene, or in limited light, or in other inspection situations wheretactile feedback is needed.

FIG. 21 shows notionally a low-cost display 2100 using LEDs(light-emitting diodes) or other luminous components 2101 arranged toindicate both the azimuthal and polar angles of the source. The display2100 comprises multiple LEDs 2101 in circular arrays with thenon-illuminated LEDs 2101 shown in stipple, and a single illuminated LED2102 is shown clear. The display thereby indicates the azimuthal angleaccording to the angular position of the illuminated LED 2102, and alsoindicates the magnitude of the polar angle according to which circle theilluminated LED 2102 is in. For example, the successive circles of LEDsmay correspond to a polar angle being small (such as 3 to 20 degrees insome embodiments), medium (21 to 50 degrees in some embodiments), orlarge (51 to 90 degrees in some embodiments). Also, a central LED 2103,shown in diagonal hatch, can be illuminated when the detector axis isaligned to the source within a predetermined limit (for example, 2degrees or less according to some embodiments).

In order to provide a more precise indication of the angles, multipleLEDs 2101 may be illuminated at once. For example, if the azimuthalangle is 22.5 degrees, two LEDs 2101 corresponding to azimuthal anglesof zero and 45 degrees may be illuminated at the same time, and theoperator would understand that this indicates 22.5 degrees. Likewise twoof the LEDs 2101 in different circles may be illuminated in a radialfashion to further refine the polar angle indication. Also, when thedetector axis is aligned with the source location, all of the LEDs 2101may be flashed together to further indicate the alignment.

FIG. 22 schematically shows a flat-screen display 2200 that indicatesboth the azimuthal and polar angles of the source. The display 2200 caninclude a rotatable asymmetric icon 2201 (such as an arrow or wedgeicon) which points in the azimuthal direction of the source relative tothe detector axis, and may also have a length or other feature thatindicates the size of the polar angle. The operator can then see fromthe shape and orientation of the icon 2201 how to move the detector tolocate the source. In addition, when the detector axis is aligned withthe source, a non-directional icon 2202 (such as a circle) can bedisplayed or prominently modulated instead of the directional icon 2201.Two other widgets 2203 and 2204, such as bar displays, can indicateother information such as the current radiation level or the integrateddose received or the presence of neutrons in the detected radiation.

FIG. 23 schematically shows the appearance of the light beam 2005 ofFIG. 20, for a version in which the light beam 2005 can be aligned withthe detector axis and point toward the source. The light beam 2005 canbe shaped as a wedge and oriented to indicate the azimuthal angle of thesource relative to the detector axis. The wedge shape may also be variedin length to indicate the size of the polar angle of the source. Severalbeam-spot FIGS. 2300-2303 are shown as they would appear at variouslocations around a source. The source location is shown as a plus-sign2304. Optionally, each beam spot 2300-2303 can have a graded intensityto more clearly indicate the direction. In the sketch, the differentbeam intensities are indicated by different stipple densities, withheavy stipple indicating the brightest beam. Thus the bright point 2308contrasts with the lower light intensity at the wide end 2309 of thebeam spot 2301. Alternatively, the gradation can comprise a colorvariation, with for example, blue at the tip 2308 and fading to red atthe wide end 2309. The operator can easily perceive a directionindicated by each beam spot 2300-2303 from the width and orientation ofthe beam spot 2300-2303 as well as the bright-to-dim intensity variation(or color variation) 2308-2309.

As the detector is moved around and the source angles are recalculated,the beam spot 2300 can be responsively adjusted to indicate the currentazimuthal angle of the source (according to the direction of the wedge)and the polar angle of the source (according to the width of the beamspot). For example, a shorter beam spot shape such as 2302 can indicatethat the polar angle is small, while a longer shape such as 2301 canindicate that the detector axis is still far from the source. When thedetector axis is brought into alignment with the source 2304, the beamcan become a circular spot 2303, thereby clearly showing the operatorwhere the source is located. Using the modulated beam spot 2300, theoperator can quickly scan for a radioactive source without looking awayfrom the scene. The modulated beam spot 2300 may be most easily visibleindoors, or inside a truck or shipping container, or under an awning, orelsewhere away from direct sunshine; however, by increasing theintensity of the beam spot 2000, and optionally flickering or otherwisemodulating it, the beam spot 2000 can be made quite visible even indirect sunshine.

FIG. 24 is a notional sketch in perspective and partially cut-away, of asystem to continuously scan pedestrians passing through a walkway orhallway according to some embodiments. The system is useful at anairport or border crossing or other passageway where large numbers ofpeople are to be scanned continuously for radiation sources. The walkway2400 is demarked by a floor 2402 and a ceiling 2401. People passing inboth directions are indicated by arrows. Above the ceiling 2401,numerous copies of the directional detector 2403 may point downward,while another set of directional detectors 2404 can be arrayed below thefloor 2402 and pointing upward. Together, the directional detectors 2403and 2404 can detect and localize a source on a particular pedestrian.Preferably, the arrays of detectors 2403 and 2404 continuously feed datato a facility computer (not shown) that analyzes the data against amoving-source model. While each detector 2403 or 2404 may detect justone or two extra gamma rays, the combined data from all the detectors2403 and 2404 clearly shows a source location as well as the directionof travel. Security cameras 2405 can continuously record the scene sothat the person carrying radioactive material can be uniquely identifiedby correlating the images with the directional data from the detectors2403 and 2404.

FIG. 25 is a notional sketch in perspective, partly cut-away 2502, of amobile area radiation scanner 2501 incorporating the directionaldetector 2503 according to some embodiments. The area scanner 2501 canbe a vehicle such as a trailer or a large van, enclosing an arraycomprising a large number of the directional detectors 2503. Preferablythe detectors 2503 are of the double-ended type as shown in FIG. 11,which simultaneously scan for hidden sources on both the left and rightside while the area scanner 2501 is in motion. The directional detectors2503 may be spaced apart to minimize mutual shadowing, although this maynot be a major concern because a hidden source is unlikely to be buriedin the road in front or behind the area scanner 2501. However, if theweapon really is hidden in the roadway, the lowest rank of detectors2503 can clearly reveal the threat when the area scanner 2501 passesover it.

The directional detectors 2503 may be configured for gamma ray detectionor neutron detection according to some embodiments. Alternatively, someof the detectors 2503 can be of each type for simultaneous scanning ofboth gammas and neutrons. The directional detectors 2503 can detect anddetermine the location of a clandestine source of radiation, which canthen be investigated in a secondary inspection. Very high sensitivitycan be obtained by analyzing all of the detectors 2503 in parallel, sothat a weak or well-shielded source can be detected even when eachdetector 2503 detects only a single particle above background. The arrayas a whole can yield a statistically significant increase, consistentwith a particular source direction. It is not necessary to make thedetectors 2503 rotatable, because each detector 2503 can determine thedirection to the source from a single orientation. Also, the motion ofthe area scanner 2501 itself can provide a range of viewpoints as ittravels through the area, so that the source can be localized in threedimensions by triangulation or by fitting to a source model for example.Optionally, a sheet of scintillator 2505 (such as plastic scintillator)may be mounted on the ceiling to reject cosmic rays according to someembodiments.

FIG. 26 is a sketch of a drive-through vehicle inspection systemcomprising two side columns 2601 and a central column 2603 according tosome embodiments. Each side column 2601 may contain an array ofdirectional detectors 2602 such as that of FIG. 2, while the centralcolumn 2603 may contain an array of the double-ended detectors 2604 suchas that of FIG. 11. A first truck 2605, which does not contain a threat,and a second truck 2606, which contains a shielded nuclear weapon 2607mounted near the ceiling, are being scanned simultaneously. Radiationescaping from the shielded weapon 2607 is detected in the detectors 2602and 2604, which then calculate the direction of the weapon 2607 relativeto each detector 2602 and 2604. The calculated directions are indicatedby arrows. Using data from the entire system, a central computer (notshown) can use triangulation to determine the exact three-dimensionallocation of the weapon 2607, including which vehicle has it.

FIG. 27 is a sketch of an alternative drive-through vehicle and cargoscanner, in which a system that detects nuclear weapons using cosmicrays is improved by the addition of directional detectors. The cosmicray system measures the tracks of cosmic rays, which scatter in acharacteristic way in the heavy shielding and the high-Z core of theweapon. Thus the cosmic ray system is intended to detect the weapon orits shield by measuring the scattering angle of the cosmic ray trackabove and below the cargo.

The facility of FIG. 27 includes an overhead compartment 2701 and anunderground compartment 2702. Each compartment 2701 and 2702 may includea cosmic ray tracking detector 2705 and 2707 which measures cosmic raytracks and calculates the scattering angle of the cosmic rays. Inaddition, an array of directional detectors 2706 may be mounted in theupper compartment 2701 above the upper cosmic ray tracker 2705, andpointing down. A second array of the directional detectors 2708 may bemounted in the underground compartment 2402 under the lower cosmic raytracker 2707, and pointing up. The directional detectors 2706 and 2708may all be positioned entirely outside the cosmic ray tracking system2705 and 2707, thereby avoiding any interference with the cosmic rayscattering measurement. A truck 2703 containing a shielded nuclearweapon 2704 is scanned by both systems simultaneously.

Cosmic rays (primarily GeV-range muons at sea level) easily penetratethe upper array of directional detectors 2706 before entering the uppertracker 2705. According to some embodiments, any scattering of cosmicrays in the upper array of directional detectors 2706 will have noeffect at all on the cosmic ray measurement, because the cosmic raymeasurement is sensitive only to scattering that occurs between theupper and lower tracker chambers 2705 and 2707. The cosmic ray trackers2705 and 2707, meanwhile, do not interfere significantly with thedirectional detectors 2706 and 2708, because the cosmic ray trackers2705 and 2707 typically comprise light, low-Z, non-hydrogenous materialswhich gamma rays and neutrons would readily pass through. Therefore, thetwo systems can operate simultaneously and independently with nodegradation in performance of either system due to the presence of theother.

Synergy is a big advantage of the combined system, with directionaldetectors 2706 and 2708 and with the cosmic ray tracker system 2705 and2707. If an adversary tries to reduce the emitted radiation signature byadding more shielding around a weapon, the cosmic ray scatteringsignature is increased and the weapon 2704 can be more easily detectedby the tracking chambers 2705 and 2707. And if the adversary tries toreduce the cosmic ray scattering signature by reducing the amount ofshielding, the directional radiation detectors 2706 and 2708 can moreeasily pick up the escaping radiation. Thus the combination of the twosystems leaves an adversary with no design space for avoiding detection.

A directional detector as disclosed herein provides numerous advantagesnot previously available in any practical prior-art detector. Accordingto various embodiments, the directional detector enables importantapplications ranging from cargo inspection, to walk-through portals, toportable survey meters, to mobile scanners searching for clandestineweapons in an urban environment, and many other critical applications.Using just a single set of detection data, the detector can indicate thesource location in two dimensions, for neutrons or gamma rays or both.The detector can determine both the azimuthal and polar angles of thesource relative to the detector axis, using only a single acquisition ofscintillator data acquired at a single position of the detector. Highdetection efficiency can be provided because the shield is nearlysurrounded by the scintillators. Light weight can be obtained since theshield is only thick enough to isolate the rod scintillators from eachother, rather than blocking incoming particles. Since the scintillatorscan be mounted closely proximate to the shield, they reduce mass,eliminate wasted space, and sharpen the signal contrast. The shield canbe shaped to reduce unnecessary weight, while still providing effectiveisolation of the rod scintillators.

Advanced radiation detection systems like that disclosed herein will beneeded in the coming decades to protect innocent people from the threatof nuclear and radiological terrorism.

The embodiments and examples provided herein illustrate the principlesof the invention and its practical application, thereby enabling one ofordinary skill in the art to best utilize the invention. Many othervariations and modifications and other uses will become apparent tothose skilled in the art, without departing from the scope of theinvention, which is defined by the appended claims.

The invention claimed is:
 1. A detector for detecting particles from aradioactive source, comprising: a shield, a panel scintillator, and Nrod scintillators; the panel scintillator and the rod scintillators areconfigured to detect the particles; the rod scintillators are positionedsymmetrically around a detector axis that extends centrally from theback to the front of the detector; the panel scintillator comprises aslab-shaped scintillator oriented perpendicular to the detector axis andpositioned frontward of the rod scintillators; the shield comprises Nshield plates oriented parallel to the detector axis, each shield platebeing configured to block most of the particles incident orthogonallythereon; the detector is configured to calculate, based at least in partupon data related to detection of the particles in the rodscintillators, an azimuthal angle of the source; and the detector isconfigured to calculate, based at least in part upon data related todetection of the particles the panel scintillator, a polar angle of thesource; where N is an integer having a value of at least
 3. 2. Thedetector of claim 1, wherein: each shield plate abuts all of the othershield plates at the detector axis; the shield substantially abuts thepanel scintillator on the back surface of the panel scintillator; theshield is configured to substantially prevent the particles from passingfrom any one of the rod scintillators into any other of the rodscintillators; and the rod scintillators and the panel scintillatortogether substantially surround the shield such that only edges of theshield plates are exposed.
 3. The detector of claim 1, wherein: theshield protrudes frontward beyond the rod scintillators by a protrusiondistance; and the protrusion distance is related to a lateral dimensionof the rod scintillators.
 4. The detector of claim 1, wherein the rodscintillators extend rearward substantially beyond the shield.
 5. Thedetector of claim 1, wherein: each rod scintillator comprises a rightprism shape; the longest dimension of each rod scintillator is orientedperpendicular to the panel scintillator; each rod scintillator isadjacent to two of the shield plates respectively; and each shield plateis adjacent to two of the rod scintillators respectively.
 6. Thedetector of claim 1, wherein: the panel scintillator is configured todetect at least 10% of the particles incident orthogonally thereon; andthe panel scintillator is configured to allow most of the particlesincident orthogonally thereon to pass through the panel scintillator. 7.The detector of claim 1, wherein: the panel scintillator has two lateraldimensions perpendicular to the detector axis; and each lateraldimension is at least four times the thickness of the panelscintillator.
 8. The detector of claim 1, wherein: the panelscintillator has an average interaction distance comprising an averagedistance traveled by the particles in the panel scintillator beforedetectably interacting therein; the panel scintillator has two lateraldimensions perpendicular to the detector axis; each lateral dimension isat least two times the average interaction distance; and the averageinteraction distance is at least two times the thickness of the panelscintillator.
 9. The detector of claim 1, wherein: the detector furtherincludes a second panel scintillator comprising a slab configured todetect the particles; the second panel scintillator is orientedperpendicular to the detector axis and is positioned behind the rodscintillators; and the detector is configured to compare particledetection rates in the panel scintillator of claim 1 and the secondpanel scintillator, thereby determining whether the source is in frontof or behind the detector.
 10. The detector of claim 1, wherein thefront surface of each rod scintillator is beveled at a bevel angle ofbetween 30 and 60 degrees relative to the detector axis.
 11. Thedetector of claim 1, wherein the panel scintillator extends laterallybeyond the rod scintillators.
 12. The detector of claim 1, wherein: thepanel scintillator comprises N panel scintillator portions, each panelscintillator portion comprising a slab positioned orthogonal to thedetector axis and frontward of the rod scintillators; and each panelscintillator portion is adjacent to exactly one of the rod scintillatorsrespectively, and each rod scintillator is adjacent to exactly one ofthe panel scintillator portions respectively.
 13. The detector of claim12, wherein the rod scintillators are optically isolated from the panelscintillator portions.
 14. The detector of claim 12, wherein: each panelscintillator portion is optically coupled to one of the rodscintillators respectively; and each rod scintillator is opticallycoupled to one of the panel scintillator portions respectively.
 15. Thedetector of claim 12, wherein the shield extends frontward beyond thepanel scintillator portions.
 16. The detector of claim 1, wherein: theshield includes a transparent portion which is transparent to lightemitted by the panel scintillator; the transparent portion is configuredto block most of the particles orthogonally incident thereon; and thetransparent portion is optically coupled to the panel scintillator andto a light sensor.
 17. The detector of claim 1, wherein the detectorincludes non-transient computer-readable media containing instructionsto perform a method comprising: determining particle detection rates ofthe panel scintillator and of the rod scintillators respectively;subtracting, from the detection rate of each rod scintillator, thedetection rate of a diametrically opposite rod scintillator, therebydetermining a differential for each rod scintillator; calculating theazimuthal angle of the source by interpolating between thedifferentials; dividing a function of the differentials by the detectionrate of the panel scintillator, thereby obtaining a ratio; and comparingthe ratio to a predetermined angular correlation function that relatesparticle detection data to particle directions, thereby determining thepolar angle of the source.
 18. The detector of claim 1, wherein thedetector further includes a light beam emitter configured to emit alight beam in a direction according to the calculated azimuthal andpolar angles of the source.
 19. The detector of claim 1, wherein thedetector further includes a light beam emitter configured to emit alight beam shaped to indicate both the azimuthal and polar angles of thesource.
 20. The detector of claim 1, wherein the detector includes acamera configured to produce an image centered on the calculateddirection of the source.